How do truss rods work?

A truss rod is a metal rod (or, on modern designs, two rods working against each other) running through the neck, and its entire job is to counteract the forward pull that string tension puts on the neck. Without one, every steel-string guitar neck would eventually bow forward into an unplayable mess.

The basic mechanism

String tension pulls the neck into a slight forward bow (called relief) over time. The truss rod runs opposite to that pull, and tightening it bends the neck backward slightly, counteracting the string tension. Loosening it allows more of that natural forward pull to show through. It's a tug of war between the rod and the strings, and your job when adjusting it is finding the right amount of tension so the neck settles into a small, healthy amount of relief — not perfectly flat, not excessively bowed.

Why "flat" isn't actually the goal

A little forward relief (generally a hair's width, measured around the 7th–8th fret with the string fretted at both ends) gives the vibrating string just enough clearance in the middle of its arc to avoid buzzing against frets, since a vibrating string's widest point of motion is in the middle of its length, not at the ends. Zero relief works for some players and setups, but many guitars buzz without a small amount of it.

Single-action vs. dual-action rods

Single-action rods only counteract forward bow — they can straighten a neck but can't actively pull it into back-bow if needed. Dual-action rods can push in both directions, which gives more flexibility for necks that have developed a back bow.

Why you shouldn't just crank it

Truss rods have a limited range and can be damaged — or damage the neck — if forced past their limit or adjusted incorrectly. Small, incremental adjustments, checked after the wood settles for a day, are the right approach. If a quarter turn hasn't done anything and you're tempted to just keep cranking, that's the moment to stop and have someone actually look at what's going on.