Simon,
It's because your main should still be way out. Traveler centered, mainsheet out, and then, as Rick explained, you sheet in the main WHILE you are making the turn to go upwind, so the sail is coming in at the same pace as the angle of the boat to the wind is changing. If you bring the mainsheet in BEFORE you start the turn, the sail will be stalled at the beginning and eventually you will reach the point in your turn where the sail is no longer stalled, and WHAM!

Sorry, I had to answer you because Rick is out of town until next week.