Bear with me on the subjective interpretation of yet another "accepted golf term" that does not say anything. I think your buddy means something else, but used the catchall term "release the club head", and that does not help.
My interpretation of the word "release" does not mean anything else, but to stop controlling the shaft of the club, to allow the club head to go on its own. This may or may not improve your swing or game. Depending on what type of golf swing someone is using, some combination of muscles are used to move the "club head". The position of the shaft of the club*, is controlled by the fingers/hands/wrists/elbows/arms/shoulders/torso/hips/knees/etc. The position of the shaft is also being controlled by the angles of your setup, and how they maintain or don't maintain those angels during the swing. So ... a lot is going on that can change the club head. The club head can't move itself. Something has to move it. I guess if you stop your body and arms midway, and let your wrists relax, the club head might move on its own. I'm going to assume your buddy means, use that back hand and throw that club head into the ball just before impact ... or "release it" into the ball, so you can add yardage. You may or may not add yardage depending on how you hit or miss-hit the ball, now that you're drastically changing things just before impact. With my method, we're after repeatable direction and solid shots for predictable distance. After we've turned through, the club head releases, unless we purposely hold it off for a particular shot (great shot to have BTW), but we don't want to mess up impact by adding "throwing the club head into the ball" to get a few yards (or not). It is too bad that TV and selling products are fixated on Distance ... like that's how to shoot par. The point here is to evaluate what your goal is ... "hit it straight or hit it far" and what is more important at the time. If you're someone that can throw the club head into the ball and pickup yardage AND keep it straight, and that is your goal to hit it as far as possible, then go for it**. * The club head is attached to the shaft, so the shaft controls the club head and we hold the shaft, so I'll talk in those terms. ** I'll bet Jon Rahm wishes he hadn't released the club head on the last hole, last weekend ... he flew the green and it cost him the tournament (and a bunch of FedEx points).
Move Less ... Get Good! DUPLESSISGOLF