haha. Really,unless you have an absurd number of extra links ,the underside tensioner works. As you pedal the underside of the chain remains taught and you don't risk tossing the chain. When you apply backpressure to skid, the tensioner moves downward out of the way.
The chain above the stay goes as slack, but not a whole lot, and gravity is helping to keep the links on the chaniring. Assuming a good chainline, most conversions with vertical dropouts lose chains at the bottom of the cog as the loose chains swings back and forth in a moving drivetrain.
It's not a "perfect" system, but vertical dropouts aren't a "perfect" foundation for fixie conversions either. It just happens to work perfectly in the real world.