Well you have to suck AND blow to make it work.
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But really its a knack of timing that's required, not unlike a chop hop. You shouldn't try any big hard pumps until you have a feel for it.
Its common to use pumping just to unstick the board and just lift it out of the water to overcome the initial resistance to planing. Once its up it doesn't take much wind to keep going. Unless you're training for the Olympics you shouldn't bother with pumping unless there's a chance of planing. Those guys are super athletes who can pump better and longer than John Holmes. There must be a minimum wind speed for the Olympic races but they have tight schedules and so they often have to sail on their Wheeties.
Typically for a pump start you do big moderately hard but slow in frequency pumps and as you pick up the amplitude gets less and the frequency increases. Like a pelican taking off. Usually you don't need more than 2 to 4 pumps to get going.
To learn it start with your feet in the middle and forward and then bend in the middle a bit pushing the sail out and forward so its balances on the mast foot but leaning away and at the same time poke your bum out so you stay balanced. Then straighten up pulling in with straight arms as you straighten your legs and push against the fin driving the board forwards. Then stop and do it again.
Pretty soon you'll be able to string a few pumps together. The timing comes naturally as you get used to it.
