I've heard this expression a couple of times before. It seems related to the engines, but I can't find any information on it, except in the context of testing them.
The space shuttle throttled down its main engines from the normal setting of approximately 104% to around 67% as it was passing through the region of max dynamic pressure ("max q"), to make sure that the certified dynamic pressure limit was not exceeded. Once the threat had passed, the engines throttled back up. If you plotted throttle level vs time on a graph, the throttle down region looks a bit like a bucket, hence the term.
See my answer to this question for some specific numbers from a particular flight.
Edit: a graph showing the "bucket", from here.