Re-engaging the operator.
On a liftmaster chain drive or belt drive: Line up the inner and outer trolley.
Pull gently on the red handle and release. (The spring needs to be short)
Press the button on the wall to run the door and reengage it automatically.
On a liftmaster or Moore-o-Matic screw drive operator, Try to line up the inner and outer trollies and pull the handle back away from the door and up until it clicks. Then press the button on the wall to operate the door.
On a genie screw drive, bring the door to waist high and pull the handle back to reengage it.
On a genie you don’t want the door at the end of the travel when you reengage it because it might be confused which direction it is going and travel right past the limits to crash up or down.