I imagine some of this comes from the sudden acceleration as the spin powers up and you end up with a rapid rotation of the apparent wind. But even under normal circumstances what is the reponse to a spinnaker collapse? Does one adjust the spinnaker trim? Alter the boats heading? Both at the same time?
2 different things happening with this question.
1) If (when) the spin does collapse over sheet to fill it back up and immediately let it back out to the proper trim. Over sheeting is very slow
2) When flying a spin down wind you are trying to play the apparent wind. If you find that you are accelerating and having any variation in the apparent wind dirrection, especially enough to collpse the spin, you have gone way to far. You may stear what feels like a relatively erratic pattern, but the apparent wind direction should not vary. This is especially critical in double handing in the light stuff as you can not feel it in the tiller when varying direction to try and power up. My crew has to tell me when they feel presure changes in their sheet as the acceration may not be immediate. If it is blowing where you are wild thinging it down wind you still only want to come up till there is an increase in presure. Comming up too high to try and get up the hull often means you went to far too fast. Somethings are about patience and the less you can move the tiller the faster you will go.
