When your getting a blow-job and as your about to cum, you hold the girl's head down and suddenly shove your cock deep into her mouth. This results in either her tonsils getting crushed, or even better a full throat penetration. At this point her head is held down as a sticky load coats her tonsils or otherwise is blasted down her throat.
The key to this move is the surprise factor. The girl receiving the "tonsil surprise" must not be expecting the move, thus she will be quite surprised as the move is suddenly executed.
In order to achieve the maximum surprise factor, the blow-job is allowed to occur at a gentle pace, with minimal thrusting. A slow build up is desireable, followed by a sudden thrusting into the mouth at the point of ejaculation. The back of the girl's head is typically held down firmly with both hands, to ensure maximum depth of penetration and to prevent the girl from being able to pull her head away before the ejaculation is complete.
I was getting some nice head last night from that chick I met on Friday night. She was totally shocked when I gave her the "tonsil surprise".