In the U.K. Anti mask contacts consist of two reeds (for U.K. Grade 3 and above).
There are two reeds, so if you put a magnetic on the contact or you try to fool the contact with a magnetic it pulls the wrong reed and generates a "mask" condition. For this reason there is a finite gap which needs to be adhered to. This change is signalled via EOL.
Im unsure if the same is true in the US?
I guess this is similar to Magnasphere.
The same is true for flush mount or roller door contacts. Interestingly if the distance on the roller door is too large you must fit one at each end so you can't jack the door up and slide under.
If the door is high security its advised to fit one at the top and one at the bottom on the side ( or other combinations) why? They cut the bottom of the door and open the bottom section leaving the top contact closed! A more common practice is to also fit a PIR or dual technology (microwave and PIR) sensor behind the door.