I'll take your word 4 it. You, and a small handful of others, helped me w/ my clutch problems in your responses to others w/similar problems.
But for curiousity's snake: what is the science behind using/not using the stand idea?
Piston rings are not manufactured perfectly round, and neither are cylinders.
They must be properly broken in, or married, before you can expect the best sealing efficiency.
In order for the piston rings to properly marry with the cylinder wall the engine needs to have a load on it.
With the engine just running static in an engine stand at any speed there really is not much pressure applied to the piston. Yes you might get screaming RPM's out of it, but that is not really working the engine, just abusing it.
When the engine is under a load you are putting resistance to the piston's ability to get pushed down by the pressure from the expanding gasses created by the combustion process. These gasses seek out every nook and cranny trying to escape. They will create pressure behind the piston rings, actually getting into the piston ring lands (ring grooves), forcing the piston ring outward away from the piston and pushing the piston ring's sealing surface tightly against the cylinder wall.
When a cylinder is brand new or freshly rebuilt they are finish honed. This honing process not only precisely sizes the cylinder to the piston size, it also leaves a fine cross hatch of ridges in the cylinder finish. Quite similar the the face of a hand file. It is important that the piston ring gets worn down by this rough surface before the surface gets worn smooth.
If this does not happen, the piston rings will never be able to properly seal against the combustion pressures and you will get what is called piston blow by. The pressurized gasses will blow by the rings and down the sides of the piston ultimately reducing the available pressure to push down on the piston and make power.
There are many differing views on the whole procedure.
This is the one that I mostly adhere to and It has not failed me yet.
I did not learn the procedure from this site, but It best describes the whole gist of how and why.
FWIW I do not agree with his take on heat cycling when it comes to these cheap Chinese engines (complete cool down between runs).
http://www.mototuneusa.com/break_in_secrets.htm