I think i know what youre talking about... It does keep its tension, but when you press "Start" then it shows the regular for of it. The red and green and yellow color of the item is just to show you how much tension you have when youre playing/editing a level
the only way you can make it keep it's tension is to hold it in place with some other anchored materials. Then you can put a timer on those pieces so they break and release the tension at the right time - very costly!