When coupling, after the kingpin appears to be locked in the fifth wheel, why must the driver also visually inspect the connection from underneath?

After coupling, the driver MUST get out and VISUALLY INSPECT the connection from underneath with a flashlight, because sound or feel alone can hide a dangerous incomplete coupling. CDL combination. CONFIRM: the locking JAWS have closed completely AROUND the kingpin SHANK (the narrow part) — NOT caught on the kingpin HEAD or closed in FRONT of the kingpin (a 'high hitch'); there is NO GAP between the fifth wheel and the trailer apron; the locking handle/lever is in the LOCKED position. A HIGH HITCH (jaws not properly around the shank) may hold briefly then release at speed, dropping the trailer — catastrophic. Always do BOTH the tug test AND the visual inspection. Knowing why the visual under-inspection is essential (to catch a high hitch) is commonly tested.