Haha, my pupils occasionally imitate me saying this! It's one of the things I do nag people about.
Arms weigh a fair amount and when you spend your time holding on to them, waggling them around or positioning them high across your torso, that's going to create problems. If your neck, shoulders, chest and back are already a bit tense and tired, using lots of muscular effort in your arms is going to make matters worse.
- When you are stationary, standing or sitting, you can let the whole weight of your arms rest, or simply drop.
- When you are speaking to somebody you can gesticulate, of course, but try to stop now and then.
- When you are walking you could let your arms swing freely.
- When you are running, the arms should definitely work as your 'accelerators'.