How does the relay valve on a trailer know how much air pressure to send to the trailer brakes?

AAMVA CDL Manual, Section 6.2.6, p. 6-6

Relay valves are why a heavy combination vehicle can still brake responsively instead of lagging behind the tractor's own brakes. Understanding that the service line's pressure, not the tank's pressure, tells the relay valve how hard to brake explains graduated trailer braking both on the exam and on the road.