Hi frenchy,
The tighter the angle, the finer the edge. A "tight" fine edge can be made VERY sharp, VERY fast, but it will wear quickly. A wider angle gives a sturdier edge, but it's tougher to impossible to get a "beyond shaving" edge with a wide angle. Think tight angle = scalpel, wide angle = axe. The right angle depends on the blade you're sharpening and what you'll be using it for. When "holding the bevel" is referred to, it means keeping a consistant angle, no matter what that angle may be. If the angle varies on a given edge as you sharpen it, you get a rounded edge (not good), if the angle remains the same, you get a flat (sharper) edge. This is true whether you're sharpening a scalpel blade (15 degrees) or an axe (I like about 40 degrees). I prefer a bevel of about 30 degrees for a general use pocket knife, and a good diamond hone to touch it up with as needed. Hope this helps.

Troy