The meditation is pretty simple (on the words): sit for long periods of time without intentional movement. Long periods as long as hour, four hours, or days.
Two rules (Shinzen Young):
Don’t damage the body.
Legs falling asleep is OK---it’s not causing you irreversible damage (it does not cut blood circulation but only nerves).
How to check: after meditation you should be back to normal in a couple of minutes. If you’re not back to normal after half an hour, you’re harming your body.
Don’t get goal-driven.
This meditation is not about how long you can sit without movement. The goal is to maximize learning.