Tying a life jacket in a double bow is in theory to prevent the life jacket straps from being tugged or loosened while exiting the aircraft.
If the pax should for what ever reason then become unconscious, life jackets with only a single bow or loosely tied could ride up/move and either suffocate or drown the poor pax and with cold hands be almost impossible to retie.
If you need to "avoid" an object James Bond style simply deflate your life jacket putting your finger in the manual inflation tube. No untying needed.
A life jacket strap should be tied tightly around the waist and not the hips to ensure a tight fit and best use of the floatation aid.
....My only gripe with the CAA/FAA is their stowage.
Under seats with lots of hand luggage and a tight seat pitch makes it both very hard to reach them and very hard to security check.
Next question!
