[amsat-bb] AO-27 Question

Hi, everybody. Today at Field Day was the first time I tried to work AO-27, and succeeded. One thing I noticed, is that as the satellite came up over the horizon, I wasn't hearing anything until what sounded like a long data burst happened. Then I heard the sat come alive, with what sounded like people already in QSO. As the sat progressed, I heard a lot of operators, then the long data burst happened again, and it seemed like the sat went "dead" I know a bit about how it's scheduled (TOPR), but since this is the first time I was able to hear it well enough to actually consider it usable. What's the data burst? Is it part of the TOPR, or am I hearing something else? 73, Jim  KQ6EA