Lifting. Including a good amount of upper body.
high cadence drills.
sprinting. In addition to developing power, sprinting is all about timing and that is developed through reps. Timing is way different between an uphill vs downhill sprint and for these guys it all needs to be second nature (including coordinating with the leadout).
as others have said, a lot of these guys are built with good systems and physiology for sprinting. The bigger issue is getting them to the sprint in good position and form after 100+ miles of riding. In order to do that they spend a lot of time working aerobic zones and training like any other all arounder cyclist.
the mental aspect of sprinting cannot be understated. Putting yourself in position for a sprint is scary and takes a lot of mental focus and confidence. The guys that get on a roll in a tour, build up confidence and it gets easier and easier. A slump compounds itself due to the drain of confidence.