This is a relatively new technique that developed in response to the very small lights that have recently come on to the market like the Surefire E1B.
The technique entails putting the head of the small light between the thumb and first fingers of your weak hand so that the tailcap of the small light rests against the base of your STRONG hand’s thumb.
You then push slightly forward with the base of your thumb to activate the light.
It’s essentially the only technique that allows you to use 2 hands with the super compact lights like the E1B.
If you aren’t careful you can get the rear of the light into the trigger guard and actually launch a round downrange.
Don’t attempt to use this technique in real life until you’ve trained EXTENSIVELY on it.