Why do rock climbers shake their arms?
Rock climbers shake their arms as a way of recovering during a route or in between routes. The purpose of this is to reduce the lactic acid build up in their forearms, which is referred to as being pumped.
There is quite a bit of controversy over the effectiveness of shaking your arms to help recover as there doesn't seem to be a definitive answer regarding if it actually works, and how. Many believe that it is better to shake your arms while they are above your heart or head, while many believe that it is better to do it when your arms are down beside you. Some also believe that shaking your arms doesn't have any effect at all, regardless of where or how you shake them.
Many climbers use a combination of the two methods, they shake their arms above their heads, and below, to their sides.
Some also believe that the effects of shaking your arms are mainly psychological, and that they don't have any physical effect or benefit.