alright, it's quite easy .... you have a sandwich of two trigger_gravities and a func_wall between them on which the item stands
[23:17]
the lower trigger_gravity has the value -1, the upper one 1 (edited)
[23:17]
you have also a trigger_always which toggles the func_wall. (edited)
[23:19]
check the spawn flags in the trigger_always to  toggle and start on and give it the func_wall as target (edited)
[23:21]