Michael, you beat me to the punch, but I was thinking the same thing.

If you consider looking down the spine of the blade the axial direction, then opening the blade past 180, or below it for that matter, produces a slight moment (twisting force) along the axial axis. If the angle is greater than 180 degrees, then the effect further down the blade is to reduce torque applied to hold the razor to the strop. The opposite is true for an angle <180 degrees.

This effect is very obvious if you open the blade a good deal. Try opening it to 90 and then stropping. This effect, especially with heavy scales, will really twist the razor in your grip. In fact, at this angle, you also have the rotational inertia of the scale mass to deal with, so upon the beginning of the stroke, the blade will tend to come completely off the leather if the initial pressure is low. At +/- 20 degrees from 180 degrees, this second effect is inconsequential.

- John