Taken from “CrossFit Total” by Mark Rippetoe.

Here are some basic precautions that need to be followed for safety:

1) Don’t be stupid.
Don’t total if you’re injured to the extent that a total will aggravate the problem.  This will cost you in at least training time, and possibly time off of work if you’re ultra-stupid.

2) Don’t be greedy.
Learn to recognize the difference between greed and ambition, and be merely ambitious.

3) Don’t be pig-headed.
If your first attempt tells you that you need to lower your second, do so, without a misplaced sense of diminished self-worth.  It’s a test, and it’s designed to measure what’s there, not create something that’s not.  That’s what training is for.

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And, a video about breathing during lifts.