The lift is object template 12 and the bobbing block template 4. Both keep their state in bit 2 of +D, which the kill bits leave free: clear while it goes down, one unit a turn, and set while it goes up, two a turn. Going down, it turns round when it is stopped (bit 2 of +C, set by the collision code whenever a move in Z is stopped, down or up); going up, it turns round when its Z passes the top, LIFT_TOP, the variable stage 1 called UNKNOWN_5B25, which the first of them updated after the room is built sets and every lift and block in the room then shares.

The lift (entry here) waits on the floor: LIFT_TOP is its first Z plus 48, and it moves only in a turn the robot or another mover of graphics 16 to 47 lands on it (bit 3 of +D, set by ADJ_DZ_FOR_OBJ_INTERSECT and cleared here), or while its last move was stopped in Z -- as it is each turn it pushes a rider up. Left alone in mid-air it would stay where it is (by the reading; not tried). The bobbing block (entry at BOBBER) moves every turn, and LIFT_TOP is its first Z: it starts at the top.

Going up, a stopped move is tried again at a step of 4: the collision code passes the lift's Z step to what it pushes into (ADJ_DZ_FOR_OBJ_INTERSECT), so the rider is lifted.

Measured in the simulator. Room $23: two lifts at Z 64 stayed put; dropped on one, he rode it up one unit a turn to Z 113 (LIFT_TOP 112), and it came down with him one unit a turn to the floor, and up again. Room $36: two bobbing blocks, LIFT_TOP 100, each falling one unit a turn to what is under it at Z 76 and rising two a turn to 102, half a cycle apart.