\nThe following is by my understanding.<\/p>\n
\nA couple of lifetimes ago, I took a heavy-duty pre-app course, ten months. Until then, I’d spent my life in the academic world. I learned a great deal in that heavy-duty pre-app program.<\/p>\n
\nYet, from any course one takes, there might be leftovers – things one didn’t quite catch, but partially recalls. For me, one topic from the heavy-duty pre-app course was transfer cases.<\/p>\n
\nA transfer case conveys power to the end of the vehicle where it’s optional. Therefore, a typical 2WD vehicle wouldn’t have a transfer case, but just an engine, transmission, then differential.<\/p>\n
\nWith a 4WD, the transfer case contains what one might call a pressure plate that turns with the output shaft to the permanent driving axle. The optional driving axle, via a clutch (that’s what I’d call it), can be shifted into contact with said pressure plate. Then, 4WD mode is accomplished.<\/p>\n
\nThe fact that it’s a clutch-type connection that actuates 4WD explains to me why, on the vehicles I’m familiar with, one can launch into 4WD while already driving.<\/p>\n
\nSource:<\/p>\n