I work on truck sized garage doors.
The spring is broken and fully unwound. You can tell because of the white line spray-painted on at the factory while it's unwound.
Doors are actually balanced.
People with garage door openers often have never operated the door by hand. They think the power opener lifts the full weight of the door by brute force.
Not at all. Doors are counterweighted so that if properly maintained, you can move the door with your thumb from any position to any other position, and it will just stay there. If it doesn't stay there it's broken. Capische?
So you have a counterweight. Down, the door is very heavy. But as the door rolls up, it gets "lighter and lighter" as part of it goes horizontal. So it must be a variable-weight counterweight. Fortunately, springs are good at that.
There is lethal "stored energy" in a door.
One way or the other, there's a ton of energy stored.
- If the door is down, the spring is wound tight.
- If the door is all the way UP, the spring is slack but the door is ready to drop like a guillotine!
It's the same amount of energy.
I never unwind a spring without raising the door fully and screwing some bolts and nuts through the track so the door can't come down. Then I'm dealing with an unwound spring with only a few pounds of force on it.
If you need to service the spring with the door down, i.e. because of a spring replacement like here, that's adventurous because you'll be messing with the full force of the spring. You can do it with winding bars if you have 2 of them and they're right fitting (NO sloppy-doppy 1/2" bar in a 5/8" port in that cheap pot-metal casting; if that thing cracks the spring releases explosively). I don't recommend that for the novice; getting spring tension right is tricky and you don't want to over-crank a spring beyond its spec.
And watch those lifting wire spools at both ends. If slack appears there, they just love to hop right off the spools, at which point the door will drop violently. It works as long as the door and spring are the only 2 things acting on that shaft. A fool with a winding bar, or a badly designed opener, can make a guillotine.