Whoa, I'm not done yet! Here are the mods you can use in In the Groove. Some mods (such as Robot and Vivid) are only available on ITG2 and beyond (not ITG PC). Here's a screenshot.
Now that we're done with the basics, let's talk about modifiers. To access the Modifiers, hold the Start Button after selecting a song (pushing it twice is also an option). There is a catch in modifiers, however - if you use these modifiers below marked with *, your score will be invalidated (the words "Will Disqualify for Ranking" is shown on your side of the screen).
Speed Modifier
Use this to adjust the speed of the arrows on-screen. This can be useful when spacing out the arrows so they can be seen better. This doesn't change the playback speed of the song.
x1: Default.
x1.5: x1.5 speed
x2: x2 speed
x2.5: x2.5 speed
x3: x3 speed
x4: x4 speed
x5: x5 speed
x6: x6 speed
C300 and C450: Change the scroll speed to mimic a song. Arrows will auto-adjust speed during tempo changes to keep a constant speed throughout the song, ignoring all BPM changes.
Many players will ask about disqualification on C300 and C450 even though I didn't mark the *. It's because it will only invalidate the high score if you use it on songs with tempo changes and stops (such as Euphoria, VerTex, and the Disconnected series). However, I noticed Hybrid had tempo changes at the end, but it was displayed as 185 BPM. Using C-Mod in this song doesn't invalidate the score. I'll ask Danford why that doesn't occur.
Perspective
This changes how the scrolling arrows approach the target arrows.
Overhead: Standard arrow scrolling perspective.
Hallway: The arrows change perspective as they scroll upwards, starting out in the distance and approaching the foreground. Default setting for ITG2/ITG3.
Distant: The perspective is tilted so that it's normal-sized at the bottom and further away at the top.
Incoming: Arrows scroll as if they were getting closer to the screen as they reach the top.
Space: The opposite of Hallway; the arrows start out in the foreground and move into the distance.
Notes
This changes how your arrows look.
Metal: Standard arrow style that changes according to the rhythm of the song.
Cel: Cel-shaded arrow style that changes according to the rhythm of the song.
Flat: Turns all arrows into orange colored arrows regardless of the beat.
Robot (ITG2 and beyond only): Turns arrows into metallic notes.
Vivid (ITG2 and beyond only): The arrows become rainbow as they scroll across the screen.
Scroll -
This changes how the arrows scroll. With none on, the arrows will scroll upward. You can turn on any number you wish.
Reverse: Arrows scroll downward and the target bar is placed near the bottom of the screen.
Split: Arrows are split up. Left and down target arrows are at the top and the right and up target arrows are at the bottom.
Alternate: A combination of Reverse and Normal arrow scrolls. The left and up targets remain at the top of the screen, and those arrows scroll up. The down and right target arrows are located at the bottom, and the arrows scroll down.
Cross: The left and right arrows are at the top of the screen, and the up and down arrows are at the bottom of the screen.
Centered: The target bar is located in the center of the screen instead of the top.
Acceleration:
This adds sudden speed changes while the arrows move.
Accel - The arrows move faster as they approach the target bar.
Decel - The arrows slow down as they approach the target bar.
Wave - The arrows slow down and speed up at fixed points on their way to the target bar, created a wave-like illusion.
Expand - The arrow spacing stretches and compresses in a rhythmic manner as though the speed modifier were increasing and decreasing.
Boomerang - The arrows enter the screen from the opposite direction, then slow down, change, and "boomerang" back up to the Target Bar.
Bumpy - The arrows appear to "bounce" toward and away from the screen, as if they were progressing over a bumpy terrain.
Effect:
Other effects that are designed to mess with your head.
Drift - The targets do not remain stationary. They move left and right, though they remain vertically aligned with each other. Scrolling arrows are adjusted as well to compensate this motion.
Dizzy- The arrows spin around as they scroll up.
Mini - The arrows and the target bar are significantly smaller than they are normally, allowing you to see more upcoming arrows.
Flip - The arrow columns are flipped. Instead of Left, Down, Up, Right, they are changed to Right, Up, Down, Left. In Double, the leftmost arrow on the screen will be the 2 Player right arrow, and vice versa. This is said to be the hardest modifier in the game.
Tornado - The arrows take a helix-like path to the target bar.
Float - The targets do not remain in a row; they move up and down, and do not remain synchronized with each other. Scrolling arrows are adjusted as well to compensate this motion.
Fade:
This adjusts the visibility of the on-screen arrows.
Fade In - Arrows do not become visible until reaching the halfway point on the screen. Note that on Marathon courses, there are variations that will cause the arrows to appear later or earlier, depending on the strength.
Fade Out - Arrow become invisible once they reach the halfway point on the screen. Note that on Marathon courses, there are variations that will cause the arrows to appear later or earlier, depending on the strength.
Blink - The arrows appear and disappear as they scroll up at a fixed interval.
Invisible - The arrows are never visible. Memorization is the key to this modifier.
Handicap:
This makes the songs easier and more manageable to play for beginners.
No Mines* - Removes all mines.
No Holds*- Removes all holds steps.
Simple - Removes all steps that do not begin on a solid beat.
No Jumps* - Converts all rows that have two or more simultaneous steps to have only one step.
No Hands* - Converts all rows that have three or more simultaneous steps to have only two steps.
People are wondering why Simple doesn't have a disqualification. This is countered however, by getting a low score even if you get all Excellents. Take Euphoria on Expert, for example.
Turn:
Changes the step chart of the song by having arrows trade places with other arrows.
Mirror - Rotates the steps 180 degrees. Steps that were up are now down and vica versa. Steps that were right are now left and vica versa.
Left- Rotates the steps 90 degrees counter-clockwise. Steps that were up are now left, steps that were left are now down, etc.
Right - Rotates the steps 90 degrees clockwise. Steps that were up are now right, steps that were right are now down, etc.
Random - Each direction is assigned randomly to a different direction. Thus, a particular arrow pattern of "left, left, down, up, right" could become "up, up, down, left, right."
Blender - Each arrow is individually reassigned an orientation. Thus, a particular arrow pattern of "left, left, down, up, right" could become "up, right, right, left, up."
Insert Steps:
This adds arrows to the existing step chart.
Stream - Inserts eighth note steps in between existing quarter note steps.
Quick - Inserts sixteenth note steps in between existing eighth note steps.
Skippy - Adds an extra sixteenth arrow right before a quarter note step, creating a pattern that causes the player to gallop.
Echo* - Adds extra eighth note arrow in the same direction after a quarter note.
Wide - Adds more random jumps, mostly of adjacent arrows (up + left, up + right, etc).
Stomp - All non-hold quarter notes become either up + down jumps or left + right jumps.
Insert Other:
Adds other arrows to the existing step chart.
Planted* - Convert some existing tap steps to be hold steps. New hold steps require at most one panel to be held at a time.
Floored* - Convert some existing tap steps to be hold steps. New hold steps require at most two panels to be held at a time.
Twister* - Convert some existing tap steps to be hold steps. New hold steps require at most three panels to be held at a time.
Add Mines - Mines are randomly added to the steps in place of other arrows. If a mine is hit, the user's score is decreased.
Hide:
This hides various score-keeping elements on the game play screen.
Hide Targets - The target arrows are removed from the screen.
Hide Judgment - The combo counter and the individual step judgments (Fantastic, Excellent, Great, etc.) do not appear on-screen.
Hide Background* - The song's background video is blacked out.
Oh, and in case you're wondering, I took these from the ITG manual directly. I hope this helps.
First-timer thread/ITG FAQ 
