To test the trailer emergency brakes, after charging the system and confirming the trailer rolls freely, what should the driver do next?

AAMVA CDL Manual, Section 6.5.2, p. 6-14 ("Test Trailer Emergency Brakes")

The trailer's emergency (spring) brakes are the only brakes available if the trailer separates from the tractor or the supply line fails on the highway, so this test is the driver's one chance to confirm they actually apply before relying on them. A driver who skips it could find out during a real breakaway that the spring brakes never set, leaving the trailer with no way to stop.