Used by the routines at LOSE_FOOD_EIGHT, EAT_FOOD, UPDATE_KNIGHT, LOSE_FOOD_THIRTY_TWO, PLACE_PLAYER and MUSHROOM_DRAIN.

The life force in $5E28 is drawn as the roast down the right-hand side, eaten away as it falls. $5E29 remembers the level the picture was last drawn at, and both are shifted right three times before being compared, so nothing happens until a whole eighth of the roast has gone -- most calls return at the third instruction.

It does not have its own copy of the picture. It reaches into the sprite tables, moves the roast's entry at ROAST_SPRITE forward by however many rows have been eaten and shortens the height bytes of GFX_B5 and GFX_B4, the second byte of each, to match, draws, and then puts all three back from the stack. The tables are only wrong for the few hundred T-states it takes to draw.

Watched live: at rest ROAST_SPRITE holds GFX_B5 and GFX_B5's height byte holds $1E, and breaking just before the restore catches ROAST_SPRITE reading $C522 and then $C51C as the roast goes down -- the same entry, advanced past the rows that have been eaten.