How do you tell when you're close to the ground?

An old blind man was describing his favorite sport - parachuting. When asked how this was accomplished, he said that things were all done for him: "I am placed in the door and told when to jump. My hand is placed on my release ring for me, and out I go." "But how do you know when you are going to land?" he was asked. "Well, I have a very keen sense of smell and I can smell the trees and grass when I am 300 feet from the ground", he answered. "But how do you know when to lift your legs for the final arrival on the ground?" he was again asked. He quickly answered, "Oh that?" The dog's leash goes slack!"