An ongoing issue in our practice is that sharpening should be done with a downward stroke. We agree that the sharpening stone should be moved in an up and down plane, but the difference lies in placing pressure on both the up and down strokes equally. In my experience, that would lead to faster wear of the blade. Another colleague believes that moving the stone off the instrument would change the plane and lead to incorrect bevels. Another difference lies in the method used for moving the stone to the three different sections that need to be sharpened: toe or tip third, middle third and heel third. I would maintain that the stone should be lifted off the cutting edge and moved to the new section, however, some prefer to slide the stone to each new section obliquely. Would that not create a strange bevel also?
You are correct that the sharpening stone should be moved in an up and down plane at a 100 to 110 degree angle from the face. Pressure needs to be applied only on the downstroke to maintain one direction of the wire edges. It is possible to remove more metal than necessary if you have equal amounts of pressure in both directions. The stone should always remain in contact with the instrument to maintain accurate angulation along the cutting edge. Moving the stone forward to reach all sections (heel, middle and toe) should be done with the stone in constant contact with the cutting edge to maintain consistency across the cutting edge. I personally do not encourage moving it forward as I believe operator pressure and angulation errors can result in a pointed toe when sharpening universal curets. Overall, maintaining constant stone to cutting edge contact would result in the least amount of inconsistency.