Rotating floor trap

[Goonie]

Im making a trap that after step on it, floor start to move and to not fall down survivors(Left 4 Dead) must balance it by standing in right place. Its something like this:



Its wood solid and nodraw texture with entity func_once.
So if survivors stand in all four corners they will not fall down. If stand in one corner(look pic: 1 will go up/2 will go down) of the floor it start to move down and if stand in two oposit corners it will start to move down to left/right. Its something like trap from National Treasure 2. I have idea how to do it and its possible but I cant figure why when change directions of floor its start to push me off and instanlty falling down

Of course bots will fall and whole campaign is point to play with players not bots.