Hold the button on the exterior, just above the license plate. Tapping it will open with power, holding it will just unlock so you can open manually.
If you see the door closing on his tail, grab the bottom of the door. The pressure will trigger the obstacle sensor and stop the closing. I don't think his tail would be enough to trip the sensor alone though.
I end up opening and closing my gate manually all the time. The power lift is great if you have a handful of items....but not great if you're in a hurry.
bare with me, i'm still practicing computer.