Cover a specific boat if you need to beat
that boat. Don’t forget about
the rest of the fleet, though.
Two boats engaging in covering maneuvers can be slow, and the rest of
the fleet can easily sail by.
If you’re in the lead, you should usually be covering those behind you
– otherwise they may find better wind.
The concept of herding the fleet (or a
single boat) involves loosely covering you’re opponent when he’s going
the way you want and tightly covering him when he’s not.
See http://www.sailingworld.com/from-the-experts/tactics-and-strategy/staying-ahead-on-the-final-beat-48736.html
for a more detailed description.