Advanced Placement

For those with tough beards, finger pressure is not an elective. Applying tension
 to the skin on the wrong side of the blade diminishes cutting power.


Power, in physics, is the application of work over time. In shaving, it is the product of blade velocity relative to skin, multiplied by the difference in skin tension on either side of the blade. This description originates in belt drive transmission, so remember the top diagram when they roll out the steam engines on the 4th of July. 

I'm sure you're familiar with a more cynical definition: the ability to influence people, e.g., to buy razors that don't work at all.


One fingertip, pulling skin opposite the direction of the razor, has the power to deshroud the mystery at the core of contemporary shaving. "Pressure" can be dismissed from the vernacular immediately, as it is seen to contribute to cutting power behind the blade, through the action of the top cap, but to deduct from power when applied ahead of the blade, through the safety bar. The companion term, "angle," is resolved as the pitch that suspends positive tension at a safe cutting point, in the hair. (Well, okay, not so much "resolved" as implied, by my amazingly skillful drawing.)

Nearly every existing tutorial on straight razor shaving shows the neck skin being stretched toward the clavicle, and a scything pass following the same direction. They are all wrong. The purpose of "stretching" should not be to make flat terrain ahead of the blade, for that is actually, physically counter-productive. The skin should be as slack as possible ahead of the blade, with not so much as a contorted expression firming the flesh. Watch a barber closely, and you may catch them pushing skin toward the blade.

The math does reflect common sense, once you've thought about it. Consider that soft flesh can bend around the edge, unbroken, while hair is inherently firm, and "caught" like a fish on a hook when forcefully driven into the edge by skin traction. Pull skin the wrong way, though, and hair more likely does a limbo dance to escape.


That's all well and good for high blade exposure and painstakingly slow shaving with short strokes and extreme angle selection. Fast-flying, long-stroking, low-exposure TTOs, as well as Techs on pick-up patrol, can address the same physics another way: delivering the power as a series of high-velocity impacts. I find this useful for fine, less firmly rooted hair, missed by the skin tension approach, and whisking away stubborn shadows at the conclusion of a shave.

One reason motion compensates for incorrect cutting angle is that the material on one side of the edge is displaced disproportionately, through conservation of momentum. Our mental microscopic view becomes a ballistics-like, slow-motion movie with shock waves. We lose some of our steam up front, as traction is distributed to the safety bar, but the principle is the same.

Even for some cartridges, loathe as I am to admit it. I think where it ultimately breaks down, in modern design, is the flexible hinge. Work will unavoidably be diverted from the edge to the pivot, turning the cutting head like the passive pulley at the top of the diagram, no matter what technique or configuration of foils is applied. You could term that as the power of M-arketing, or con-Fusion.

The next disquisition elucidates a more compelling reason why the single blade allows precise separation of hair and skin, when dynamics are considered in three dimensions.