Fly sparging, also called continuous sparging, adds hot sparge water to the top of the grain bed at the same rate wort drains out the bottom — the grain bed stays fully submerged and gently rinsed throughout the whole process, rather than being drained completely and re-flooded the way batch sparging works.
Matching two rates by hand is the real skill here
The entire technique depends on keeping inflow and outflow rates matched closely enough that the grain bed neither runs dry (risking channeling and a stuck lauter) nor floods and overflows — traditionally managed by hand with careful attention through the whole runoff, though some homebrew setups now use an adjustable sparge arm or a simple float valve to automate the balance.
The efficiency case for the extra attention
Fly sparging typically extracts marginally more sugar from the same grain bill than batch sparging does, since the grain bed is rinsed continuously and gradually rather than in one or two large exchanges — a real, measurable difference, though usually modest enough that many homebrewers judge batch sparging's simplicity a better trade for a normal-strength recipe, reserving fly sparging's extra efficiency for situations where every point of yield genuinely matters, such as a very high-gravity batch.