Go to http://www.youtube.com/watch?v=WMvralRauSE and watch for the
sequence BEFORE the rescuer attaches to the 'blue' bag. Youll note
that he wraps 2 loops around the descender for extra heavy loads ....
youll only need 'one' loop around the descender to make it work for a
boom brake device.

I use it on a 440 sq. ft. mainsail. (with one loop around the
device). Why I use the 'eared' type of descender is that the 'ears'
stop the line from overwrapping around the descender, thus jamming.
I tie a line to the gen track, up to the descender affixed to a boom
bail, then to a block on the opposite genoa track ... then back to the
cockpit. I control the amount of friction (hence the amount of
frictional amount applied by tensioning the line (via a cam cleat).
When you set up, do a few trial gybes until you get the tension on the
line correct ... which sets the amount of friction desired.

:-)