If you are talking about preloading the suspension which I think you are then here is how it works. Don't go and get killed, I only have 1 year of MX racing myself. Preloading is in my mind generally reserved for small jumps about log size. These jumps when hit with momentum tend to have the rear suspension kick up and make you land very nose first. For these type jumps I try to do this. Get in jump position, standing etc. and about 3-5 feet before the jump push down on the bars hard and instanly go back to normal stance. This will hopefully cause the front end to be unloading while going up the face of the jump. This should counter what the rear end trys to do naturally and give a neutral position while in the air.
Like I said, this is what I use it for, my one bud does this for every race and is a expert MXer with years of experience.
Hope this helps.
Stan