The strap works like this. Say for example you are pulling out your friend, so you attach your recovery strap to the back of your vehicle and then to the front of his. As you drive away from him the recovery strap stretches just like a rubber band. The strap wants to go back to its normal size, so it has no other choice than to transfer the energy to the stuck vehicle, pulling it out.
Recovery straps are flat with sewn loops and no hooks. They are nylon, not polypropylene or Dacron or chain. Because the straps are made of Nylon, they stretch. They are safer than chains, easier to use, and not nearly as heavy.