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Lynx, unlike MS, will read two keys held simultaneously in the process of moving as moving 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 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 (more generally, which keystroke is considered primary depends solely on the direction Chip is currently facing, so sliding can also reverse priority). The second situation is that if there is a block in the subordinate direction, block slapping can occur.
When the direction Chip is facing and a perpendicular direction are held on a tick when Chip is fully on a tile, a block is next to Chip in the perpendicular direction, and Chip is able to enter the tiles in both directions normally (even if deadly), the block in question will be moved one space in the subordinate direction, as if it had been flicked. This is more easily done in constant motion, in which the primary direction is likely already held down and the player only needs to start pressing the subordinate direction sometime before reaching the block, rather than make sure they press two directions at the same time.
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.
Note that, because Chip never enters the tile underneath the block, said tile remains unchanged; for example, items would remain uncollected, and locks would remain unopened. The latter example is an interesting case, because while Chip needs the corresponding key, he retains the key after the block slap. Also note that real blue walls and hidden walls are considered enter-able until revealed, even though they can never actually be emtered. This allows blocks to be flicked off those tiles almost exactly like in MS.
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.
In the original Lynx game
Block slapping in the original Atari Lynx version of Chip's Challenge is strictly more flexible than in Tile World's emulation. How exactly it is more flexible is not well known.
In Chip's Challenge 2
As in Lynx, the ability to slap a block depends on whether the player can enter the tile underneath. For example, Melinda cannot slap a block on gravel without hiking boots, and Chip cannot slap a block on a female-only sign. Unlike Lynx, however, CC2 does not consider real blue walls and hidden walls enter-able. To emulate the Lynx behavior in CC2, such walls must be replaced with recessed walls—and even then, level designers must be wary of spring mining. (To emulate block-on-recessed wall Lynx behavior in CC2, a level designer can place a block on a thief and give the player an item that must be kept. If the item must be a tool, the designer can place no signs, which can be empty, in the relevant areas.)
Block slapping in CC2 is strictly more flexible than in TW Lynx, but less so than in Atari Lynx. More precisely, in addition to the slaps possible in TW Lynx, if the player is not directly facing a block that is directly above or below, they can slap the block while moving left or right.
If the player has a hook, then any time the player can block slap, they can also hook slap, provided that pulling the block normally is a legal move and that the block to be pulled is after the player in monster order (which usually means the block needs to be placed before the player in reading order before the level starts).
A variation of block slapping in Chip's Challenge 2 is wall slapping. If Chip or Melinda moves past a series of walls while holding a second key in their direction, the wall will be slapped. This can reveal hidden walls and invisible walls, clear fake blue walls and expose real ones, and apply steel foil to walls. However, revealing a wall and converting it to steel do not happen on the same frame by the same movable object, so converting a real blue wall to a steel wall, for example, without directly touching it requires two slaps.
A deadly variation occurs when holding a second key while walking past a monster. If Chip or Melinda does this without a helmet, they will instantly die and have to start the level over. This is also present in the Atari Lynx version.
Monsters can be hook slapped. This is usually not detectable, since hooking a monster does not directly kill the player, but can be seen when hooking the monster on the last possible tick if the monster is about to move toward the player and the monster is after the player in monster order, in which case the player will be treated like a wall and the monster will try their next direction instead.
In official levels
Block slapping is required to solve Two Sets of Rules when playing in Lynx. It is also required to solve several Lynx-only levels in CCLXP2. In addition to this, several other official levels, including Zartacla and Reticulating Splines, are busted due to block slapping.