Vladimir,

I'll describe the Mainsail boom, the Mizzen boom is similiar. 

There are two block and tackles with cam cleats (like a typical mainsheet on a smaller boat), one each side of the boom.  One end hooks to the boom just forward of the mainsheet, the other hooks to a stainless steel loop at the forward end of the jib sheet car near a stanchion. 

When running you tighten the leeward preventer.  When jibing loosen the leeward preventer, using it to ease the boom over.  When the jibe is complete tighten the preventer on the (new) leeward side.

You have to go out on deck to pop the line out of the cam cleat, I'd rather it be operable from the cockpit.

The "Boom Brakes" are friction systems that connect to the boom with a line going through it and fastening at each side of the boat where the preventer would.  The idea is that it's mostly a hands-off preventer, once you have the tension adjusted in the brake for the current wind speed.

Duane
Wanderer, SM#477