In short the Beta stands for "B" which stands for Breakage.
When any file is ground, it starts out as a cylinder of metal, is then ground into a cone and then flutes are ground into the cone.
If the flutes are far apart, then the groove that creates the flute must be ground deeper to create the flute.
In some files these grooves actually go past the center of the cone.
The smaller the interflute distance the shallower the grooves and thus more metal to add strength and break resistance.
Fine-Cut Files have the smallest interflute distance of any file on the market today.
And thus have the greatest strength at their core.
The favorable Beta Factor and the low helical angles make Fine-Cut files almost impossible to break unless they are buckled or twisted, which if the proper technique is followed, allow a lot of leeway to push, pull, probe and cut without stressing the file anywhere near its breaking point.