Floor kick

A floor kick, like a wall kick, happens when a player rotates a piece when no space exists in the squares where that tetromino would normally occupy after the rotation when rotating against the floor opposed to a wall. To compensate, the game sets a certain number of alternative spaces for the tetromino to look. Floor kicks first appeared in Tetris X game because 1G made some tetrominoes difficult to manage.

Rotating a flat-side-down tetromino against a flat floor often requires a floor kick.

Examples from the bounding box style rotation system used by SRS:

Note that floor kicks in SRS are diagonal!

Games using TGM rotation do not need the same floor kick for T as described above because the J, L, S, T, Z normally rotate to keep a constant bottom edge. However, these games need floor kick in a different circumstance where bounding-box systems do not:

TGM1, TGM2, TAP do not perform floor kick. TGM3 in classic (TGM rotation) mode does so only for I and T. Official Scratch Tetris allows any piece floorkick.

Abuse
In many games, when the player rotates the active tetromino to kick it up one square, rotates it back, and lets the tetromino fall and land again, the lock delay resets. This prevents the tetromino from ever locking and prevents the player from ever topping out. The CPU opponent in The New Tetris has been seen to do this.

For this reason, some games restrict the number of floor kicks performed per tetromino. Tetris DS in multiplayer mode will lock the tetromino after a few seconds. TGM3 allows only one floor kick per tetromino:

Constant floor kicks
In Tetris Best 2, constant floor kicks are possible, as shown below.

The gap is required, because for rotation to even be done, down, up or left kick must succeed (clockwise) or down, up or right (counter-clockwise).

After the kick is done, the piece returns to original location.

Then it floorkicks. This is why TB2R has 2 consecutive wallkicks.

Note that the last three images scroll up the playfield.

This originated in Tetris Best as preventing getting stuck in blocks. Because Tetris Best 2 rotation may overlap blocks, this is in effect floorkick.