In my opinion, applying full pressure prior to beginning the cure cycle is a poor practice. It defeats some important beneficial effects of the autoclave process. It was primarily motivated by the desire to test the integrity of the bag, so that if it is going to burst, it does so at a time when the cure cycle can be aborted and the bagging repaired. The rationale of being able to process different resins in one cycle may also apply, but I've not seen this done.

The reason for not applying pressure until after heating is to allow the volatiles in the resin to expand the residual air in the prepreg and cause it to percolate out to the vacuum, before compaction pressure is applied. Many think of the compaction pressure as squeezing air out of the laminate. However, it simultaneously compacts the laminate and pinches-off the the paths through which air can be removed. Volatiles expand by changing from liquid to vapor state and will provide orders of magnitude greater expansion than simply heating air. This is very effective at removing residual air. Then when pressure is applied, the remaining volatiles (with no air) can be condensed and dissolved into the resin.