If your lather is a "frothy mass of soap and water" then the lather will have very little cushion. Air provides neither slickness, nor cushion. It is true that adding water to your lather does increase slickness, at least until the point of breakdown. If your lather is too dry, it will have less slickness, but will provide more cushion. I try to get a happy balance between slickness and cushion. If the lather is somewhere between Greek Yogurt and sour cream, it is probably right.I have found the same to be true with DE razors - I always ride the cap and try to use the shallowest possible angle.
Must say though, I never grasped the concept of lather "cushion". I mean lather is a frothy mass of soap and water bubbles. How in the world is that going to cushion a steel blade from cutting into skin?
I prefer a very wet, thin, almost transparent lather, as I am looking for extra sleekness and lubrication for an easier glide. Cushion? I don't get it. I see folks posting pics of meringue like foam mountains, but I don't see the point.
@RayClem any comments?
My test for a properly hydrated lather is to fill my sink with water, leaving a slight trickle. Then as I shave, I gently dip my razor into that water. If the lather sticks to the blade, I need to add more water to the lather. If the lather immediately leaves the blade and dissipates, then I have too much water and need more soap. If the lather is properly hydrated, it will quickly release from the blade, but float to the surface of the water largely intact. Some of the better soaps have a very wide range of acceptable hydration so your can tailor the lather to the balance of slickness and cushion you desire.
Think about a grease in a bearing. The grease provides a thin layer of lubrication that separates the parts of the bearing, even under extreme pressure. The metal parts should never contact each other; otherwise there will be rapid wear. There is a cushion between the metal parts. It does not have to be a thick cushion, only one microscopically thick. A great lather will provide a thin layer of lubrication and cushion between the edge of your razor blade and your skin. For that to occur, the lather has to be the proper viscosity/thickness, just as as motor oil has to be the proper viscosity to protect the engine. If you put lightweight mineral oil in your engine, it would not last long. If the oil is too thick, you might not even be able to start the engine. For similar reasons, it is critical that your lather has the proper level of hydration top achieve the proper viscosity.
I have found a relationship between residual slickness and cushion and post shave conditioning. A soap can have superb primary slickness, but have poor residual slickness. However, with the very best soaps, I have found that I can perform my clean-up pass without re-lathering my face. I can do that in spite my sensitive skin. I do not dare try that with lesser soaps, however. When I am using such a soap, I can do a four-pass shave with zero skin irritation. That thin layer of lather that remains on the skin protects, moisturizes, and conditions.
There are folks with tougher skin who indicate that they do not want cushion in their soap. They believe the very thin layer of soap on their skin makes it more difficult to achieve BBS. That may well be the case. I do want to achieve near-BBS as my beard grows rapidly. That is why I do four passes. However, my priority is to complete my shave with zero irritation.