i don't use it. i prefer to do a long warm up, at least 30-35 minutes, and definitely nothing all out, though you should throw in a few hard efforts. get your HR up, but don't tax yourself too much; that's for later.

afterwards i do two 20-min efforts, the first one not all out, the second one all out. If say i get 310 and 320 for the efforts, i'll take 310 and call it a day. You'll know how good your protocol is based on the ease with which you complete your subsequent workouts.