Nice!
The valve timing by modded/vented hydraulic lifters? to get fast flow over/past the chamber nozzle/vent is there to see.
Then there's the shaped piston head we wont get into now. that spreads that flame front radially and horizontally.
So the fill/scavenge process is like blowing over the head of a bottle to make it 'sing'
and the richer mixture is due to the fact that at or around TDC; the piston is hardly moving, making for a richer mixture at that time, as the fuel injection rate remains constant for the stroke.
Then the piston moves away (picking up speed) and the 'blow over the bottle head' effect goes away, leaving the rich charge in the chamber...
ie: Without valve overlap and scavenging and a good banana branch or turbo this wont work!
Hmmm-OK! Yes the penny seems to be dropping at last!
I assume the chambers were loaded with a spray gun for the video?