The HCG diet seems to be all the rage and admittedly havent done a lot of research into it but from what I know, its not meant for people actively performing strenuous exercise, so that basically rules out any BB or weightlifter cutting. As for how it works, you cant help but lose weight when only eating 500 cals a day. The only thing that comes to mind is the HCG causing an extra test boost in men to keep mass loss to a minimum. But going back to this is not for active lifestyles then those folks who this diet is meant for wont really have a lot of mass to worry about anyway.
A zone diet and crossfit peels the fat off like nothing I have seen. Your not going to get massive doing CF but if your goals are cutting you can probably keep your mass AND lose a lot of fat in 30-60 days. Do a CF workout 5 days a week and on 3-5 of them spend about 20 minutes working on a single heavy compound lift. Do about 4 warm up sets then a 5,3,2,1,1 rep scheme for the heavy compound movement. Compound movement should be bench, deadlift, squat, overhead press or an oly lift. That should be enough work to keep your size and strength and the high intensticy cyclic and anerobic nature of the crossfit WOD will melt fat off you.