The "inner core" of the tape measure has a small piece of steel in it that acts like a spring. Another way to think about it is that the little wire on the trigger of a toy pistol that "hits" the bullet bends when it is pulled out, and when it is released the force (the pulling force, which is less friction than the elastic force of the small wire) disappears, so it springs back. And the part of the tape measure that has a fold in the front, that's where it comes in, and it keeps the tape stuck on the outside, so it doesn't completely shrink in. It's loose at first, but when it's pulled out, it's tight, and when you let it go, it should have a healing process, so it bounces back. It is that the inside is loose at the beginning. When it is pulled, it is tight. There is a spring between it and then it will naturally spring back.
