The bypass technique splits your brew water into two parts: most of it goes through the coffee bed during brewing, and a smaller portion (the bypass) is added directly to the cup at the end without ever touching the grounds.
Why bypass
The technique solves a specific problem. When you brew very lightly roasted coffee at a standard 1:16 ratio, you sometimes need a finer grind and more contact time to extract enough flavor. But finer grind plus longer contact often pulls out unwanted bitterness alongside the sweetness.
Bypass lets you brew at a stronger ratio (say 1:13) for better extraction, then dilute the brewed concentrate with the reserved water to bring the strength back to drinkable. You get the extraction benefits of a stronger brew without the heavy mouthfeel.
How to do it
For a 1:16 brew with bypass: 18g coffee, 240g brew water, 50g bypass water reserved. Brew with the 240g (a 1:13 ratio in the cup), then add the 50g bypass to the finished coffee. Total liquid is 290g, the same as a standard 1:16 brew, but the extraction profile is different.
When bypass helps
Light Nordic roasts, dense washed Ethiopians, and very fresh coffee that is hard to extract evenly. If you find yourself fighting underextraction (sour, thin cups) on a particular bag, try bypass before changing equipment or technique drastically.
When bypass hurts
Skip it for medium and dark roasts. They extract easily and bypass dilution can make them taste flat. Skip it also when you have already dialed in a coffee at a standard ratio and like the result; bypass is a problem-solver, not a default upgrade.