I've seen people trap limbs and not even realize that they were doing it.
Funny how this subject comes up now...
This weekend I competed in a tournament.
During the match, my opponent threw an "blind" spinning technique. He was trying for a backfist but smacked me with his elbow right in the face.
This happened to be the SECOND time he had smacked me with excessive contact with a blind technique (blind meaning, I was looking at the back of his head and he couldn't see what he was swinging at), the first incident having happened about a year ago.
So about a minute later I bridged the gap on him again, ending up with him turning his back to me again.
Without thinking, I used my left arm to pin his left arm up against his body while I gave him a hard shot to the head
Observers told me about it later, but I had to watch the tape before I realized what I had done
Funny thing is, my training partner ALSO did a trap-punch combination in HIS match (different division) the same day! (Also unintentional). His was a circular wrap — counterclockwise circle with his left arm to trap the opponent's arm (something right out of our technique curriculum).
I'm thinking traps DO work, if the time and timing is right. Kind of a pre-emptive block of sorts.