Shaving Brush Knot Length - Loft

The loft simply describes the length of the knot, from its base to the tip.

Longer lofts will give you more flex in the bristles, to the point where they can become floppy whereas a shorter loft with start to stiffen the bristle. 

Finding a length (LOFT) that suits you is simply a matter of experimentation. All lengths will work well if the brush still maintains good backbone.