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As I understand it the sheer energy that they can hold and suddenly release if mishandled is potentially lethal.
True of any decent-sized spring under load. There are very few tools which genuinely scare me. OxyAcetylene kit is one, chop saw is another, and spring compressors definitely fall into that category. I don't have to use them that often, but when I do it's definitely squeaky bum time.
Just to add to the garage door thread (@Dammit), are the existing doors sprung or counterweighted? I ask only because every time someone on Reddit asks, "what's incredibly dangerous that nobody knows about?" garage door springs are always mentioned. As I understand it the sheer energy that they can hold and suddenly release if mishandled is potentially lethal.