You need a speed controller with a brake that will stop the prop from spinning. If the stopped prop won't fold then the tiny rubberbands will do the job if you can position them properly. You have to install two hooks for each blade. The hooks are to be located near the hinge on the back of the folding blade and on the back of the prop hub. The hooks must be spaced to give the proper tension with the size rubberbands you choose. If the spacing is too great the tension will be too high and the prop won't unfold when you advance the throttle. If the spacing is too close there won't be enough tension to fold the prop.