I was really enjoying a thread about sprinting on cyclingforums.com for a while, though the last page or two are somewhat less enjoyable. At the risk of just saying "me too!" I would have to agree with many of the posts in which it is pointed out that when we sprint in a real race, we rarely, if ever, use 100% of our potential.
The typical "sprint finish" in a criterium or road race usually comes only after many miles of fast riding, often punctuated with attacks or climbs, closing gaps, etc. So we are pretty tired when that finish line approaches, even when we feel pretty good. I guess you could say it's the guy who's lost the least of his sprint power that wins.
My team's coach, Mark Edwards, did a test during one of our training rides in which he sprinted "100%" after we'd finished some 5-minute intervals, and had a short break. It was something like 840 Watts, as I recall. Kind of disappointing, in that he can usually hit over 1000W when he's fresh. Later on he led me out for a sprint and could only muster about 670W or so (not the exact numbers, but close enough). He tried, but that was the most his legs could manage after the previous riding.
Much of my current training revolves around that exact theory... and the smart guys race with that theory as their main tactic. I always try to minimize my efforts while racing, so I can save some energy for the sprint. Though if I get carried away with that tactic I can easily miss the winning break, and have done so too many times! Part of it is knowing when you have to "burn a match," as we say.
When I train for sprint finishes, I mostly train with long intervals; as long as 20 or even 40 minutes at my full 1-hour power (my "FTP"). That power is something like 272W, or maybe 309W, depending on who you ask. (I don't have a power meter yet so I must estimate with various calculator tools.) Does that seem weird? Ride 20-minute intervals so I can be faster in a 15-second sprint? Well, it would be weird if we arrived at the finish line completely fresh, but we don't. By training with those long intervals I will arrive at the finish line more fresh than the guys who only train with short intervals (at best), or with sprints or even weights.
OK, not everybody is the same, and some guys train for track sprints at the velodrome where they do start their sprints with fresh legs. But for the racing that I do, training just for short sprints is missing the important point.
My training does include 10-second "jumps" (sprints), and also 6-minute intervals (at my "VO2max"), but the core is those long intervals. You'll never catch me lifting heavy weights to improve my sprint!