The Lynx Chip's Challenge, unlike the MS version, will read two keys held simultaneously in the process of running as running in the first direction pressed. The second direction is a subordinate keystroke, which will take effect only in two situations. The first situation is that when during (not at the beginning of) a straight run, the primary keystroke becomes illegal but the secondary keystroke is legal, they will reverse priority and Chip will move in the previously subordinate direction. If, however, there is a block in the subordinate direction, block slapping takes effect.
When the subordinate direction is pressed after Chip starts moving in the primary direction, but before the block's row or column is reached as the case may be, the block in question will be moved one space in the subordinate direction, as if it had been flicked. Therefore, if the player is fast enough, a block one row or column away can be slapped.
In this cut from Zartacla, using Lynx rules, Chip can get block 2 off the fire space without being killed, and therefore exit the level in a total of five seconds. Chip would come onto [10, 0] playing through the level as normal, and when he moves D, within half a move the L key is also hit. The combination of this is such that when Chip moves 2D, block 2 moves L, such that block 3 or 1 links to its dirt and Chip can exit the level. Note that if the exit and hidden fire were on row 1 in the above diagram, and there was a wall above [10, 0], this would be impossible to do. However, block slapping could be performed by running vertically instead.
Why it happens
It is likely that block slapping is an ancestor of the coding which allowed for the Mouse Panel Glitch. Both behaviors involve an attempted move in one direction, such that another direction entirely is moved. Since the flick is not possible in Lynx, block slapping is its closest relative.