Why does the manual recommend holding the steering wheel firmly with both hands on opposite sides?

49 CFR 383.111(a)(4) (Basic control); AAMVA CDL Manual Sec. 2.2.2, p.2-8

A sudden jolt from a pothole or curb can wrench an improperly held steering wheel out of a driver's hands, momentarily taking away directional control of a heavy vehicle. A firm, two-handed grip is a simple habit that prevents a road surface hazard from becoming a steering control emergency.