1. The bonuses will not appear as a buff on your bar, I had to watch the characters stats screen to track the buff presence and stacking. This makes it difficult to know if the buff is still up or not until you have a good intuition for it.
2. It procs not just on the 4 spirit generators, but also sweeping wind and exploding palm (for a total of 6 abilities which contribute to the buff). Not sure if this is a bug or is intended, as sweeping wind and exploding palm used to be spirit generators before they were changed. (The sweeping wind buff is applied when it does damage, not when you use the ability)
3. There is only 1 buff to refresh, not a buff per ability. This means that once you have used all of the 6 abilities once, you can spam a single ability to keep the full 6 stacked buff up.
Most hell+ monk builds seem to include a spirit generator and sweeping winds, which means that for most off-the-shelf builds combination strike will be worth a 16% damage increase.