When a trailer's wheels lock up and the trailer begins to skid ("trailer jackknife"), what is the correct recovery procedure according to the CDL manual?

49 CFR 383.115(c) ("off-tracking, response to steering, sensory feedback, braking..."); AAMVA CDL Manual Section 6.1.5 "Prevent Trailer Skids," Figure 6.2 and "Stop Using the Brake" procedure, p. 6-3

The instinct to grab the trailer hand brake in a skid is exactly backward — it's what caused the skid. Knowing to release the brakes instead can be the difference between recovering control and a full jackknife.