I have an old Sears garage door opener, and the receiver appears to trigger the door to open or close by shorting the two wires that run to the mechanicals box. But those wires also provide the power for the receiver - in this case it measures about 30VAC, which the schematic says is a nominal24VAC off the transformer secondary.
It appears that newer boxes work the same way, and new replacement rolling code receivers also do that.
It would seem that this system would be problematical, particularly for the newer receivers that need to remember stuff.
Have I misinterpreted what appears to be going on here? If not, how/why does this work?