For those that aren't aware of what it is.

It is two lines from the Boom, at about the location of a Std Vang. One
line goes Str-B, the other goes to Port. I take each to a block mounted
on the toe rail, even with the Mast. I use a line long enough to come
from the Boom, through the block and back to the stern.

When I release the Main Sheet for reaching, I can haul down on the Boom
to flatten the sail, maintaining the sail shape I set-up when I set the
Sail.

This can and is used to act as a preventer that can be controlled from
the cockpit.

It can also be used as a Boom Brake when Gybing,

When tacking or gybing you must remember to release the other line
(TWIN)

As I did enjoy singlehanding I have both. A Std Vang and the Twin. This
allows me to maintain sail shape control from the Cockpit

I connect to the boom with a cleat and two loops. This way I can release
the lines and pull the loops to the toe rail to eliminate a tripping
hazard.

Ole Thom