Pull sets with bands, NO buoys - HOW??????

Okay, I can do a 25.  I get it, you swim hard, head way down.  I'm bottom heavy, but whatever.  I've seen some sets where there are 4X100 like that.  What am I missing here?  I'm actually a pretty decent swimmer, most of these sets are distance, presumably triathlete friendly, and I'm generally a much better swimmer than even very good triathletes (better than the few locals who make Kona).  But I simply cannot do a 100 without a buoy, much less 4 of them.

Am I missing something?  What am I doing wrong, here?