Intervals are specified by duration and effort. I do some intervals all-out, and some of them I pace, depending on the workout. For instance, all my intervals are all-out TTs for the given duration, so I will be gassed at the end. On the other hand, my hill repeats (which are essentially 3:55 intervals) are paced so that I can climb the hill a second or two faster each successive hill repeat -- but that's to train pacing more than just fitness.
Most of my intervals end with me well above LTHR.
Sprints are a different animal. They are explosive, out of the saddle, hands in the drops, laying down as much power as possible for 15s or so. I usually map out 200m landmarks (25 revs in a 53/14). Then I roll up to the line at 20mph, and sprint from there, in about my 53/15, and don't shift. Sometimes I do standing starts from the line in a 53/16 or 53/17. Sometimes I sprint off a training partner's leadout.