I am guessing here, I'm no expert with hammer effects.
I would make the log a door. If that doesn't help, here is a more detailed explanation:
put the log in a func_door (the brush can be made invisible, it's an option in the entity properties) with a direction of down and set a low speed, and put a trigger multiple (as close as possible to the log, and not under it, if that makes sense).
In this entity's outputs, set OnStartTouch -> [log/door name here] -> open
then OnEndTouch -> [log/door name here] -> close
(pretty self explanatory that last one

.
see what that does...