2 week on 4 weeks off. Bill W. has some great info about them over at his site (mesorx.com). The idea is that during the first two weeks the body does not go into complete od deep shutdown. You need to use fast acting AAS, and clomid after. The clomid is to help start you back up ASAP, as after the two week shutdown you are "hyper" sensitive and came make good use of your nat test during that time.
Most users make gain during the 3rd week when they are off.