I think training for the jump has three main parts to it. First, build the muscle strength. Most good sprinters use squats and similar gym work, but Kevin Worley, a certain AMD-Masters rider, and I do virtually no gym training for sprinting-it's all done on the bike. Kevin and I use uphill sprints (mentioned in detail below), and standing starts. It takes months and months to actually change muscle fibers and grow them, so plan to do this part of the training beginning in the Fall and continuing to the Summer.
You can do these 100% effort sprints from a start at 0-5mph, for 10-20 seconds long. Choose a gear that allows you to finish the short efforts at around 50 rpm's and the longer efforts at around 90-95 rpm's. A hill for the second half of the effort, or all of the effort keeps your rpm's from going too high (which would change the focus of the training). Keep your arms almost straight (they provide their best support in this way) with your back fairly flat. Focus on pushing down on the pedals, but also on pulling up a little too. You will get out of the effort what you put into it. I try to break the pedals and the crankarms. About 20-30% of these efforts can be done in the saddle with smaller gears and starting from about 40 rpm's. Recovery time from the most intense sessions of this type of (strength) training will normally be 5-10 days. Pilates is an excellent way to help strengthen some of the supporting muscles of the trunk and back.
This brings up the second part of jump training. You are trying to make the muscle's motor units fire quickly, and all at once in a well-coordinated fashion-that's how they produce maximum force. Training for this can be short, 100% efforts, in a smaller gear than you would normally use for racing-maybe 2 or 3 teeth more on the rear cog/cassette. I often use 53x17 on the flat or a 53x16 when starting from 25mph after a downhill. This will cause fairly high force with quick recruitment of the motor units, and will be at a cadence that is at, or slightly higher than what you normally see during your race sprints. Keep the sprints short, 8-12 seconds long, and do most of them out of the saddle. Occasionally you can do a few in a smaller gear while staying in the saddle, and this is especially good for improving your sprinting in mass-start track races. When you are doing other hard training in the session you can incorporate just a handful of these sprints for good benefit.
Normally you can do 3 or 5 sprints in a set, rest 2-4 minutes between sprints, then rest for about 7-8 minutes between sets, then do another set. When you are too tired to maintain the quickness and good technique it is time to stop. You can aim for a total of 10 sprints, and 15 would be very good. The benefits of this training can be seen after just a few sessions done once or twice a week.
You'll see a change that is similar to when people first start lifting weights. They are able to add weight to the bar almost every week for about 6 weeks in a row, but their increase in "strength" is not due to significant changes in the fibers, but in how the fibers/motor units are learning to coordinate themselves.
The third part of the jump is technique-coordinating all of your body's various muscle groups to work together to produce a good jump and sprint. It helps to have someone who knows what they're doing check
your form occassionally to help correct any major flaws, like your position on the bike, how you handle various cadences, gearing choices, etc. A major part of this training comes from practice in gears that are maybe just a little easier than what you'd normally use for racing, but sometimes you'll want to use your race gear(s). You'll want to do these sprints on the flats, up slight hills, down slight hills, starting from 10mph, 20mph, 30mph, etc.
These sprints can be more like what you'd do in races, 10-20 seconds long,
with 4-5 minutes rest between them. Maybe 2 or 3 sprints per set. Focus on
pushing down hard and also pulling up a little, especially as you sit down
into the saddle to finish the sprints. And when you sit down, do it
gradually. I picture a plane coming down to a runway.
Always finish your sprints strong, with no decrease in effort and speed. This is how you win races.