During my analysis of cmda6, I discovered that Game Freak implemented their own script-controlled walking ASM into the Advance-generation games. There can be up to eight subroutines to run on every frame of animation, only one of which may be active at any given time. You can select a subroutine to activate using cmda6.
At 0x03005090, there is a list of ASM functions to be executed on every frame of animation. Each entry in the list is a pointer to the routine, some metadata about the list item itself, and thirty-or-so bytes for the routine to work with (so that it may maintain its state).
When on the overworld, one of the items on this list is 0x0806E811, a walking routine manager. This routine manager will check one of the bytes in its execution-list-item (set by cmda6) and based on that byte, it will call one of eight walking subroutines.
Those subroutines in turn check the player's coordinates against stored values to see if the player has moved. If so, the subroutine processes player movement accordingly (check the tile they're standing on, change it if necessary, what have you).
There are eight slots for walking subroutines, and the defined subroutines (pointed to by pointers at 0x083A7310) are:
#0 at 0x0806E955: Nop
#1 at 0x0806EB55: Broken (R/S/E leftover: Route 113 ash-covered grass)
#2 at 0x0806E955: Nop
#3 at 0x0806E955: Nop
#4 at 0x0806E9E1: Icefall Cave ice tiles
#5 at 0x0806E955: Nop
#6 at 0x0806E955: Nop
#7 at 0x0806EC41: Broken (R/S/E leftover: Granite Cave/Sky Pillar broken floor tiles)
(The three defined subroutines basically change certain tiles out from under the player's feet. Theoretically, though, a subroutine can do anything it wants on every frame of animation that the overworld is being processed.)
What this means is that we now have an official way -- something that was designed for this use -- to set up our own custom-made ASM functions to run the very instant the player takes a step. If we keep the broken functions in the table, we can define up to four custom ASM subroutines; if we ditch those, we can define six.
(We could also repoint and extend the subroutine pointer table, and modify the related ASM code, thereby allowing up to 255 custom subroutines to be predefined and activated with cmda6.)
One possible use case for all of this would be an alternate (and more script-friendly) implementation of JPAN-style walking scripts, which would work without breaking other game functions (i.e. wild encounters in tall grass).
For more information, see the description for cmda6 in my FireRed script command reference.
Oh, something else: the R/S/E leftovers prove that this discovery applies to all Advance-generation games. The offsets will differ, and the walking subroutines will have some differences in R/S/E, but the system itself exists in all Pokemon GBA games.