If you enable physics on a physics item slot into a physics slot, the item won’t be “ejected” from the physics slot but will have its physics enabled without collision, causing the physics slot to still be occupied while the physics item falls through the ground. Same applies for respawning, although the respawned item won’t have physics enabled (unless enabled in the edit menu).
If you stash an item that’s currently inside a physics slot (via the item finder), the game will not only stash the physics item, but will also stash the physics slot.
If you stash a physics slot that’s currently holding a physics item, the physics item will remain, but won’t have collision and can’t be interacted with except for stashing it via the item finder (which works as normal).