Swinging your arms upward, your weight becomes heavier

When you are on a weight meter and swing your arms upward, your weight is displayed heavier than usual.
You can easily understand this phenomenon by thinking about a baggage as a substitute of your arms.
When you lift up your baggage, your legs must stand firm more than usual.
So, more power is applied to the weight meter you are on.
Conversely, if you bring something down, the ground you are on take less power by your feet.
This is explained by “reaction”.
If you are in the air and stretch upward, you also stretch down ward.
Because, a force which is “reaction” of the force your stretching occurs.

It is similar to the phenomenon in which your upper body and lower body approaches each other when you jump and pike in the air.
You are also stretched downward, so if your feet are on the ground, your feet push the ground more than when you are just on the ground.
People utilize this when you jump upward or back flip and jump higher.