Originally Posted by sailingdog
A preventer being used with out proper knowledge of how to use it can be just as dangerous as not using one. If the boat gybes, and the preventer line isn't release, it can cause the boat to get "pinned down" by the mainsail. While this may not be as dangerous as an uncontrolled boom swinging across the boat, it can cause its own share of problems, including heeling the boat over enough to have crew fall off. It also requires someone to go forward to release and re-attach the preventer lines each time you gybe—not an ideal situation for novice sailors.
That's why I have my non-standard ones rigged on both sides. It's somewhat like a boom brake or twinned vangs, I suppose, but with the cams down on deck running aft. The blue and white is starboard (they were out of green and white that day!) and the red and white is port. They can be used under tension as a replacement for the main sheet when gybing normally.
I'm aware that there are large stresses at play here and I wouldn't use this set-up above 25 knots or so, but I find it's simplified my single-handing.