It's all about feel in my opinion; When setting up on the rear wheel, get your positioning correct and also stalling before your pre-load into your rear tyre (which gives you the bounce for the sidehop). The time to release the rear brake is just the same as you would for a gap. When your in the pre-load position and your ready to kick the pedals, thats the time to release the brake, cause' obviously, if you kick the pedals and don't release your rear brake, your not gonna' go anywhere. Found this image in the wiki on here; Pre-load position is below;