This post will show you in a visual way at what angle to hold the knife and how much pressure to use when stropping. Big thanks to TwinDog at bladeforums for this post.
A lot of people have noticed that stropping can make an edge duller as well as sharper. Some say you need a lot of pressure, some say you need no more than the weight of the knife. Actually, you need to get the right combination of angle and pressure.
I tried to capture this relationship visually:
The black string represents the surface of the leather strop. The wood represents the edge. It is cut to a 30-degree inclusive angle, a popular edge angle for knives with good steel. Ideally, you want the string parallel with the edge bevel.
First: Proper angle and very little pressure on the strop. With very little pressure, the leather is not depressed significantly and dragging the edge over the strop at the sharpened angle sharpens the entire edge bevel without rounding the edge. People who say you need little pressure are using the correct sharpening angle. Notice the string is parallel with the edge bevel.
Second: This shows the angle of the knife held too high on the strop, rounding the edge. This is wrong no matter what the pressure.
Third: This shows the edge held at too shallow an angle, with little pressure applied. This combination of angle and pressure rounds the shoulders of the edge bevel without affecting the edge.
Fourth: This shows the angle held too shallow, but with a lot of pressure. The pressure compensates for the incorrect angle and the edge is sharpened. Notice the string is parallel with the edge bevel. This combination will sharpen well. People who use a lot of pressure, are using a shallow stropping angle.
Finally: This shows the correct sharpening angle, but with a lot of pressure. The extra pressure causes the stopping leathers surface to drag around the edge, dulling it.