Yup. (during install) You often use two short 1/2" steel rods to wind tension into the spring, using one to hold the tension while you move the other to the next position and/or lock down the set screws that keep it in place.
If you do this incorrectly and slip, that spring will unwind in a hurry and usually throw one of those ~5lb steel rods with enough force to put it through a wall.
One of those hits you you're gonna have a bad time.
If you can... Often the door in the up position blocks access to the springs, leaving you no choice but to do it door down. Part of the reason it's so dangerous. Wouldn't have to manually build so much tension if you could leave the door up to install them.
Yeah I did that once. Scary as fuck. I definitely kept my body off to the side and made absolutely certain that I had the bars fully seated. Still, probably not going to do it myself next time.
Garage doors have one of two types of springs in them, and both of them are capable of killing you if handled incorrectly.
One can hurl a piece of metal into you at high speeds, and the other can cut you like a steel whip.
It's one of those things where you should be firmly in the category of "definitely know what you're doing" before you consider doing it yourself.
Depends on the type of garage. It'll either be at the front, above the door and running parallel to the opening, or it'll be a big ass spring above your head when you're facing the door.
In the first case, it's possible it'll just look like a black or green cylinder at first glance.