Creating a Leash for Your Ice Axe - Spokane Mountaineers 

Slings can't stretch though; the point of this is to make sure the shock chord stretches when you want to move the ice axe away from you

Precisely. Another common question is why do this instead of buying a bungee leash. Most bungee leashes aren't rated very high, my Black Diamond one is only rated for 2kN; whereas this gives you something fully rated.