Trapping is when the bit is on the other side of the board from the fence. What can make this dangerous is that with a straight bit there is nothing to control how much wood the bit is feeding into. The board can get pulled by the bit action away from the fence and you lose control. With the bit at least partially behind the fence face depth of cut is controlled. If you are trying to make a slotted mortise into the face of the fence then I would clamp a stop on the fence and hold the end of the board against it and swing the other end of the board into the fence. You'd be able to stay in control that way.
Someone I consider a master woodworker once told me that a master woodworker is not someone who never makes mistakes. He is someone who is able to cover them up so that no one can tell.