I heard it in a recent TV episode. Someone walks with a limp (probably a ruptured Achilles' heel), and the doctor mentions that he had the accident “blowing a dismount on a keg stand”.
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"Blowing" in this sense means getting wrong, screwing up, etc.
"a dismount" is an attempt to get off something ("The part of a routine in which the gymnast detaches from an apparatus.")
I had no idea what a "keg stand" is, but Wikipedia helped me out.
Read up on gymnastics for an answer. That question if too generalized to be suitably answered on a site other then for gymnastics aficionados. Blow it is an expression meaning to fail.