dropping series of masses connected by spring

I am trying to show how a series of 2D masses connected by string will fall. I have connected 10 masses with a spring joint. However they all fall at the same time. I would like to make the simulation such that their is initial tension and then the tension at the top is released so the bottom box does not fall until the rest of the masses have collapsed. Does anyone know how this would be done? What would I say in my script?