Used by the routine at player_controls.

The first thing the player's handler (player_controls) does each frame. The variable at $5BB1 is set to 1 by toggle_day_night at every sunrise and sunset; if it is set, and the player is neither in the settling time just after entering a room (the top nibble of +$0C) nor in the middle of a jump (bit 3 of +$0C), the change from knight to werewolf or back begins now.

Starting it replaces the rest of the player's frame: the two INC SPs throw away the return address into player_controls, so the final RET of rand_legs_sprite returns straight to the object loop and the player neither reads the keys nor moves. $5BB1 now holds the player's type, which is how the end of the transformation (finish_transform) knows which way to change. The top half of the player is erased; the bottom half becomes one of the spinning transformation sprites, types 92 to 95, handled from now on by upd_92_to_95.