Half Hitch as a Bowline Backtie
A half hitch is an overhand knot tied around departing cord. It's the simplest way to secure a bowline waist loop for critical uses like climbing.
Start with a snugged-up bowline tied belt-tight around the narrowest part of your body.
Take the free end and wrap it around a departing portion of the waist loop.
Insert the free end through the resulting loop - going AWAY from the knot.
Pull it tight.
CAUTION: Always look closely at the result. The bowline should be an obvious bight of rope "choked off" by a loop to the standing end - NOT a slipknot with backtie.