Once you are comfortable with form, try doing what I did: do very few reps (like 4) with a very heavy weight. Not too heavy for you but the heaviest you can lift. If you can do one rep and that's all, then stop there.
You will feel it in the glutes and hamstrings.
This exercise is great for the entire upper body. Just training to keep stabilized, is great for everyday activities, for injury resistance. It trains you to instinctively protect your low back while moving through life. Do not plan heavy dead lifts the same week you are moving, or helping someone move (house).
Anyway the main muscles used are large, and therefore can handle a heavy weight.
I do not think actually relaxing any upper body muscles is good, just contracting them in the right direction. "Irradiation" is a term I have seen used- it means increasing your strength through contracting all the muscles around the prime mover. So you would want to contract your whole torso in its neutral position, contract biceps and triceps and forearm (grip), and retract the shoulders somewhat.
For example, for a biceps curl, you would grip quite hard, retract the scapula, contract the chest, to stabilize intensely. This enables you to lift a higher weight for more reps. Good to know when you need your strength on the job.