Here's a short YouTube video that shows a few tumbles from knife-edge in the Extra 300L. I demoed a couple, and then a friend of mine tried some, with interesting results.

For this type of tumble (a variety of Lomcevak), the procedure is:

• From straight-and-level and at full power and 120-140 KIAS, pitch up smartly to 45 degrees.
• Roll to left knife-edge (90 degree bank) and immediately apply full right rudder to hold the nose up.
• Briskly push the stick to the forward-right corner. It should be held against the stop.
• The airplane begins an outside snap roll to the right and rotates around all three axes.
• At an entry speed of about 90 KIAS, the airplane has enough energy to complete three full rotations.
• At the entry, you normally experience about -2.5G.

Note that during the demos, after the first full rotation, the airplane appears to pivot around the left wingtip. My version of turns around a point.