I saw this video on YT about this pipe freeze kit to freeze the pipes (I assume with liquid CO2) so if the shut off is broken the pipe can still be cut for service?

[www.youtube.com]

Wouldn't that damage the pipe because I keep hearing in the north when it gets cold the pipe freezes then when thawing they crack on swelling.

As long as both ends are open so the expansion is not trapped, there is no problem.

I thought about doing this. I noticed that
no store in Albuquerque sells them. I found
a deleted listing for them on Home Depot's
website. I suspect people use them dangerously.
A party goods store only a half-mile from me
sells dry ice, 75¢/pound. A pound is more
than enough for a pair of jobs on half-inch
copper, as long as you're not sweating on a
joint too close to the point you're freezing.
Slapping on a Sharkbite is easy.
Put the leftover in the freezer - may as well
not waste the cold.

Arthurpeabody, it might be challenging to manipulate dry ice into position on pipes. It can freeze the skin right off your bones. Look up handling precautions, which start with thick gloves and arm coverings.

Note that the video talks about freezing in place of "drain down" and not "shut off"....When there is pressure in the pipe and a spot freezes, the hydraulic pressure from the water doesn't let the ice expand and it will split the copper pipe.

It's not clear from this video, but as hj says, both ends must be open. So you still need to shut off the pressure, but this blocks the water from trickling in when you are trying to sweat on a new fitting.

If you are sweating a new fitting in the basement, once you turn off the supply you need to open faucets and drain 1 fl and 2 fl so the pipe is dry for sweating. On our old house in NY I installed shutoffs on the pipes going to 2 fl and fixtures on 1 fl, so I didn't have to drain the whole house to work in the basement.