In the discussions that led to PADI publishing the article on teaching while neutrally buoyant, I mentioned mask clearing as an example of a skill that has to be taught incorrectly when instruction is done on the knees. The PADI representative was adamant that the article not say it was categorically wrong to teach on the knees, so the result was the addition of the (ridiculous) statement that if skills are introduced to students on their knees, the students should then do the skills again in horizontal trim, hopefully in the same session. That is ridiculous because it says you have to teach all the skills twice, which nobody is going to do.