I'm not quite sure what you mean, but I got a pretty good idea. I think the imaging chamber floor moves as Al does, so he is not really moveing, but he is walking. Ans as for positioning, he can position his image so that lined up with the floor. That also explains why he can float, like in 'Leaping in Without a Net.'
Like I said just above, if he can line his images up with the floor, he can sit on the imaging chamber floor and make it look like he's sitting on the floor wherever Sam is. It doesn't work with a chair. When he is sat next to Sam in cars and stuff, he is not really sitting, it just looks like he is. Like in 'Leaping in Without a Net' again, when Sam is driving and Al is sticking out of the roof and Sam says 'Can't you just look like you're sitting next to me?'