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.