Basically (and i mean in simple terms) having the two pulleys adjustable means you can dial the timing in spot on irrespective of head thickness, gasket thickness etc which will of course alter the distance between the head and the block.

The Vernier pulleys have a small Vernier scale on them, then you can set the engine to tdc and rotate it to the point of max lift on one of the cams e.g. inlet and using the dti move the cam on the pulley until it shows full lift, then lock it. rotate to position for exhaust and do the same. then rotate the engine slowly by hand to ensure valve clearance check belt tension then recheck timing again.

Sounds simple, but it really not something i'd advocate doing unless:

1. You've fully understood what is going when you are timing them in
or
2. Have plenty of money to rebuild the engine if you've got it wrong

But like i said this is just a simple explanation as there is a bit more to it.

Hope that answers your question.