"...but most pozi caps just fall out!"
Most 'star' caps rely on an interference (or friction) between the sides of the driver recess and the 4 'wings' of the stem of the cap.
This only works if the screw’s drive recess is close to tolerance. In practice, on a #8 countersunk screw, tolerances can vary by 20% (even more on some imported screws), so normal ‘Pozi’ caps will (more often than not) be too loose or too tight.
The Pro-Dec Solution
Rather than making a cap (like everyone else) that doesn't work, Pro-Dec set about a radical re-design:
The Stem: Bifix caps have a patented stem, incorporating 16 tiny wings, or splines. When aligned properly in the screw-head, the splines push against each other, and against the walls of the central (circular) part of the drive recess, rather like tiny springs.
This guarantees a good grip in both Phillips and Pozidriv screws, even when screws are out-of-tolerance, burred and damaged. In fact, this makes Bifix the best ‘Pozi’ cap available – part of the reason Bifix is the only ‘Pozi’ cap to be granted a patent in the UK.
The Crown: Bifix caps utilise the same low-profile crown used on Stemfix caps. This means the cap is less noticeable and importantly, when the cupboard is being washed or wiped clean that cloths won't snag on the edges of the caps.
The underside of the crown is slightly concave to allow for screws that have not been driven flush with the work-surface. The crown is flexible to ensure the smoothest transition from the work-surface to the cap.