It doesn't seem to be specific about the exact type of exercise. Any idea on what's a good upper body exercise? Push-ups, bench press, shoulder press, etc? Some people say wrist curls are good because they build up the forearms.

I stick to mostly body weight exercises plus a weight vest. I avoid the bench press like the plague. It's a good way to get hurt. Wrist curls seem to be effective. Rock climbing has worked the best for me.