The top of a cloud is rising, building higher and higher into colder and colder air. At some point, water freezes, and begins falling. Usually it melts before hitting the ground and turns to rain. If it doesn't melt it is hail on the ground.
Under a building cloud air rises because it is being sucked up by the cloud building. When the cloud begins icing, the top starts dropping. I have seen many a cloud-top just disappear. It doesn't really disappear, it just moves earthward.
So in this theory, the falling ice is held up by the mass of rising air. So the water precipitating is one force going down and is met by the force of rising air. That rising air twists the falling water around at the bottom of the cloud making the funnel.
Around that funnel, the opposing forces involved, feed each other more force. The wind blows harder forming the water into a concentrated force which becomes the funnel. Can't have a funnel without a very strong wind. Indeed, falling rain creates wind, too.
That very strong wind, wrapped around the funnel, at ground level, is now rapidly rising with more force because of the tornado, and that is what makes us think the tornado is sucking.
Top of the cloud is now shrinking because the ice has somewhere to go - down through the funnel. And then, when the ice in the top of the cloud is gone, the tornado is gone also.
In short: Rising air stops the ice from falling. The force of the falling ice overcomes the rising air by forming a funnel. Take away the falling ice and you take away the tornado.
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