Why does my brake keep pinging out and almost killing me?
That looks like the quick release mechanism used when removing the tyre to open the caliper a little wider.

It looks broken? - it should be tucked under in line with the pull of the cable. Since it isn't the cable is actually pulling the release open as you brake.

You might be able to temporarily remedy it by leaving it in the open position and pulling the cable tighter. You would however lose the use of the quick release. Probably not a great idea to run it like this - since the cable will be pulling on the fixing nut at a strange angle.
Best replace with a new caliper.