You should try using a bungee cord.  When you pull it tight, it will still constrict even more once it’s tied in place.  Also try tying your balls with regular rope (or use a ball parachute), then attach a bungee between your balls and a steady object like a post.  When you pull on it, it has more give, and you can work up to a harder and harder pull.  Try using different strength bungees as well.  When pulling with regular rope, there isn’t nearly as much give, or room to stretch and play with.