Mijbil said:
Strop tautness. Every guy that ever paid me a visit to talk about razor sharpening, poses that question at some point.
Let's answer with another question. Do you think our ancestors were stupid enough to use a hanging strop while they could just as easily glue a strip of leather onto a wooden base? In fact, that's what they did for pasted strops, but not for clean leather.
I think the answer is straightforward. Pasted stropping still aims at abrading steel. This is done at the bevel
faces. Clean stropping aims at aligning the the erratic part of the edge. This is located
on top of the bevel. That is literally "around the corner", hence the strop needs to bend around the edge. The amount is critical: too much and the edge is smothered. Too little and you'll end up with a poorly stropped edge.
I my video, the camera angles show the amount of bend in the strop much better than what you see when you look down on the leather while stropping a razor. It's the same principle that's responsible why most women see their own boobs smaller than those of others.

A matter of viewpoint. Beware that you don't make the same mistake by allowing more slack in your strop than you might think.
I bet none of the above answers the question. In essence the strop is held taut. The pressure on the razor is what bends the strop. Slack is never guided by the hand that holds the strop. I find it very important that the strop bends mainly under the spine, and far less at the edge itself. If you get that part right, the razor will almost automatically find the right contact with the leather.
Stropping is something that's learned by repetition. I could state it is done by feel, but for a newbie such statements are meaningless, because he has no way to experience that "feel". How does a rope artist manages to stay on the rope? By repetition. Repetition is what forces our brain to transfer control from the cerebrum to the cerebellum. The cerebellum is one of evolutionary oldest part of our brain, and it is capable of amazing things, but it's a veeerryyyyy slow learner. Till the cerebellum kicks in, we'll have to do with our rather clumsy, but fast learning cerebrum. Does anyone care for an example of cerebellum capabilities?
My point is that
stropping is practice. Once you've reached that level, this tautness question will loose its meaning, because you'll be able to feel the exact ratio of pressure with one hand and pull with the other. It's something not to think about, but to just "do". It's one of the many zen-aspects of straight razor shaving.
PA23-250 said:
This, and also Ray's post, is why I have tried to describe properties of a good strop in my article. Obviously it's possible to get great results of strops that don't meet these recommendations. But let's just say a horse is not as easily tamed as a calf.
PA23-250 said:
Just write them a message in simple English.
One of the employees speaks English well.