The idea is to empty the lungs of air, and with just the air remaining in the mouth cavity, squeak in the upper register (without the use of the tongue) above high C. It’s difficult to describe, but the it feels like you’re just using the air from the ‘neck upwards’.
It’s a Donald Reinhardt approach and there’s more details in the Pivot System Manual. But that’s the jist of it.
I’ve been doing it for a while now and the squeakers are getting more stable and I’m able to get more air behind them so they develop into notes. The only problem I have is the louder I play them the more arm pressure I use.