Playing the rim blown flute
In order to make a sound on a rim blown flute you have to do three things simultaneously:
Occlude the end of the flute
Create a thin stream of air which blows directly across the cutting edge
Create an opening in front of the cutting edge to make a sound hole
The original flute from the broken-flute cave in Arizona, had only a continuous thin rim for the embouchure.
This style of embouchure was probably played either obliquely
More recent adaptations of the embrochure have included a ramp along one side of the edge. This is based on the Japanese Shakuhachi flute embouchure. This creates a shallow notch to aid in creating the sound hole. The Shakuhachi is designed to be played along the wide edge, creating variations in the sound and amounts of air.
My more recent adaptation of the embouchure has in internal ramp, based on the southern Chinese Xiao. The Xiao is a meditative instrument designed to create pure tones. This is closer to the sound we admire in the ancestral pueblo flutes,
This is a diagram of the more recent adaptations.