Bypass brewing is when part of the brewing water does not go through the coffee itself. The technique should be familiar to anyone who has ever owned an Aeropress, since the included instructions describe the process: one brews a concentrate which is then diluted. While I don't like bypass brewing so extreme as the standard (which is actually nonstandard!) Aeropress technique, I do appreciate more subtle uses of the technique.
This morning I brewed up PT's Bolivia, which is still tasting great even though it's fairly old in coffee time.
I used a flat-bottom pourover, a Kalita, and was going for a perfectly flat bed of coffee. Going for that bed often clogs the paper filter slightly, which leads to a slower draw-down time. With my grind and the temperature of my water, I didn't want this process to go much longer than 3:30. At 3:30, though, I still had about 50 mL of water to pour. What to do?
I took the dripper off the mug, and added 50 mL of hot water directly to it, bypassing the grounds. I stirred, and the result was an elegant representation of a very pretty coffee from PT's.
If I hadn't added that extra 50 mL, the cup probably would've been too strong for my tastes, and I wouldn't have been able to pick out the flavors so easily. Because I dosed 60 g/L, flowed through the coffee grounds evenly, and took the process to 3:30, I extracted most of the goodness from the grounds. Adding that last 50 mL of water simply allowed me to achieve a more balanced result.
Bypass brewing for me, then, is like a safety valve. If my brew is taking a bit too long, I end it sooner rather than later, and make up the remaining water via bypass, which clarifies the brew and doesn't run the risk of overextraction.