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Hopefully someone has done a technically competent study and can tell us. I am a bit puzzled - you say "snap on" but then note something about screws...? ?
To attach prop guards almost always requires the operator to attach each prop guard to one of the arms of the copter. The most common way of doing this is to remove each of the four screws which hold the motor in place. Holes in the prop guard are aligned with the four screw holes under the motor and are attached with screws supplies by the seller.
Doing this "in the field" is almost impossible to do without losing screws, so someone invented prop guards where all you attach to the copter is a small plastic pad under each motor. Once you have that done you simply take the remainder of each prop guard and "snap" it onto a pad using that manufacturer's locking mechanism.
One great advantage of the "snap-on" type is that you can easily and quickly put them on and take them off whenever you want.
One disadvantage is that if you ever want to remove those attachment pads, be careful which screws you employ to fasten the motor to the arm again. Screws that are used to hold prop guards on are slightly longer then those which come standard with the Phantom 3. If you try to use one of these longer screws without the prop guard being installed you can easily push the tip of the screw up into the bottom of the motor, thereby damaging it. ALWAYS keep the two different lengths of screws separate from each other (I use a small plastic self-sealing bag for each type, with a slip of paper inside describing what the screws are for.)
Hope this helps...