there are so many different types of tumbles, i forget what the true Lomcevak is, but this is how I remember it from watching a leo laudenslager video one time (I used to do this all the time with my old lanier dart).

pull to a 45-60 degree upline

Start a full deflection aileron roll to either side (one way will work better than the other....left I think)

When you get to knife edge, immediatly throw in full opposite rudder (opposite to the direction you are rolling) and full down elevator.

The plane should do a wierd outside snap and then go tail over nose....very very cool

PS. It helps to have a heavy plane with heavy wings (more roll inertia)