It's motion imparted on the intake charge both in the port areas and the chamber as the piston is in motion. It's not only produced by the tight piston to head clearance but also is affected by the lower intake tract. It's an extension of what a good port design does... Swirl is mostly produced by the port, valve job, and chamber shape. These cause the mixture to twist as it passes the valve, enhancing flow through the valve and helping ensure complete cylinder filling, plus helping to keep fuel in suspension during the intake and compression strokes. Tumble simplified is the motion imparted to the mixture as the piston moves up the bore during the compression stroke. It gets forced out of the areas between the piston and head and tumbles away from the flats and circumference to the center of the dish. The tighter the quench, the more that happens. the more that happens, the more efficient the burn can be.
BTW - the greatest leverage on the crank is not TDC. It's variable depending on the stroke and rod, but normally around 15° past TDC.
