In our training we start things quite early. For example, footwork and hand motions are taught together from the beginning, and we often use chest protectors during set drills so that we can become familiar with hitting a 'human' target with some force. When you start out, your power generation and its use is bad, as is the correct distancing, timing, etc, as people aren't used to co-ordinating all of these things together. So lots of training is required
We also train power stationary too, on the wall bag, and work stepping with striking on the heavy bag. Sometimes we do pad work too - which really shows you your problems (you moving, the other guy moving, while you're trying to a: hit the target, b: do so with good force).
Once we get to Chi Sau training then its about controlling the power when things are happening quickly. In the Lat Sau training things can get rougher, as there's more momentum involved. One thing overlooked, IMO, is learning to let go of fear. It's hard to train power with a training partner if you're in fear of getting hit (everyone is so, when they start).