Fly sparge = vorlauf, then open the valve into the boil kettle a little. At the same time add water from the HLT at a rate that the grainbed stays covered by an inch or so. Then adjust the flow rates so that it takes close to an hour to fill the boil kettle to your preboil amounts.
Batch sparge = vorlauf then drain the mash tun. Add water for the sparge in either one batch or sometimes 2 or three. Stir like crazy, let it sit for a few minutes the vorlauf again, drain again then repeat if you are doing the sparge in more than one step.
This is the simplified explanation, not covering temperatures, setting the grainbed, or the risk of channeling with fly sparging. It also does not address the different ways of distributing fly sparge water.