|This is the notation I prefer for tongue pizzicato|
To get a good POP, you have to close off your air passage from behind and in front, compress the trapped air, then release it. Perhaps a "bubble" image will help. That is what we are doing, popping air after all. It's a simple concept that each flutist can do differently. I'll now go into boring detail about what works for me.
To block the air from the back, raise the back of the tongue as if you are beginning to swallow. If you try to close your throat further down (as it is in the middle of a swallow), that won't work (for me).
For a tongue pizzicato, the release of of the air bubble can be varied, tongue on the lips, or tongue on the palate.
If on the palate, I find it more effective if the tongue is slightly retroflex. That's a fancy word, but actually it only means the tip of the tongue is behind the hard palate, pointing up but not pointed. There should be an air-tight chamber (bubble), with the hard palate as the roof and your tongue as the walls and floor. The pressure to create the pop is made by trying to push the air bubble forward. When you feel the pressure, you can release front of the tongue and let the jaw drop a tiny, tiny bit, that will help the air bubble go down into the flute.
In the pizzicato with tongue on the lips (behind or between the lips, both are possible), the bubble's roof is the roof of your mouth, the walls are the teeth and cheeks, and the floor is your tongue. The pressure is built up by squeezing whatever muscles you can (lips, cheek tongue, whatever works), then drawing the tongue quickly back. Letting the jaw drop here a tiny, tiny bit can also help here with resonance, getting the air bubble into the flute.
Hard to put into words what goes on inside us!