During the trailer service brake test, the driver moves forward slowly and applies the brakes using only the hand control (trolley valve). Why does the procedure call for the hand valve here instead of the foot pedal?

AAMVA CDL Manual, Section 6.5.2, p. 6-13 ("Test Trailer Service Brakes")

Because the foot pedal brakes every wheel on the rig at once, a driver could press it and feel the vehicle slow even if the trailer's brakes are completely disconnected, since the tractor's own wheels would still do the stopping. Using the hand valve isolates the trailer circuit so the driver can confirm, before ever pulling onto the road, that the trailer brakes are actually connected and working.