What is the correct practice for downshifting before descending a hill or entering a curve?

49 CFR 383.111(a)(5) (Shifting); AAMVA CDL Manual Section 2.3.1, p.2-9 (downshift-before-hill/curve guidance) and Section 2.16.2, p.2-32 (the specific hazard of trying to downshift after speed has already built up on a grade)

Trying to shift down after a truck has already picked up speed on a downgrade is a leading contributor to runaway-truck situations, since momentum can make it nearly impossible to find a lower gear once rolling fast. Downshifting before the grade keeps engine braking in reserve, saving the service brakes from overheating and failing. This is one of the most safety-critical shifting habits tested on the exam.