Also called continuous sparging.
In this method, after vorlauf, the wort runoff is begun and water is added to the mash tun at the same rate as the runoff. It’s important to go slow so as to extract the maximum amount of sugar and not compact the grainbed, which would stop the runoff. Lauter design is also highly important in fly sparging. Your lautering system must allow no channeling, or the sparge liquor will “drill” straight down through the grain bed in only one or 2 locations and leave the rest of the mash unrinsed. Because the buffering power of the grains in the mashtun is continually being diluted by the sparge water, it’s necessary to monitor the pH of the runoff. Too high a pH will cause the extraction of tannins and polyphenols, compromising the quality of the beer. To counteract this, it is often necessary to acidify the sparge water to keep the pH of the runoff below 6. Because the runoff may take an hour or more, many brewers do a mashout step in an attempt to denature the enzymes and prevent further conversion from taking place while the sparge is happening. However, this method will usually yield the highest extraction from the grain.
Source: denny brew