Bifix Caps are the ultimate 'Pozi' caps, that press into the drive recess of countersunk screws.
Bifix Caps fit Pozi and Phillips head screws that have a No.2 drive recess.
Bifix caps are designed for use with flush-fitted countersunk fixing
that have a No.2 drive recess (the size of the recess determines the size of the screwdriver or bit you should be using – in this case a No.2 driver).
A No.2 drive recess is usually found on M3.5 to M4.2 (No6 to No.8) screws - but less frequently on larger fixings.
If you look at the stem closely, you will see that some wings are bigger than others.
These wings are designed to fit inside the screw in a certain order,
and just like any other Pozi cap, Bifix Caps need to be aligned in the screw before being pushed into place.
When properly fitted, the micro-wings of the stem press against each other,
and against the walls of the screw's drive recess, achieving a grip that is greatly superior to the grip of a common pozi cap.
Brute force and/or hitting the cap with a hammer will NOT achieve a better fit.
It will however render the stem unusable and damage the top of the cap!