I have this opener (x3):
Guess i was confused by what they mean by a jackshaft, cz I have that. I guess that I was thinking screw shaft or chain that drives a trolley hook.
The jackshaft is spring loaded to take some of the weight of the door so the motor doesn't really dead-lift the thing. Also, you don't manually lift up on the door if the power goes out. There is a chain hanging on the side of the lift assembly that you use to turn the shaft.
These are not really DIY install. I had the local lift door guys come out and do it.