My interpretation of "getting into the groove" is that you subvocalize observations about your subject, then you relate those observation to drawing instructions (going over the surface, coming through, going across, etc). At a certain point you don't need to subvocalize and you can draw freely on autopilot. This part sounds kooky but I think its accurate to say that those instructions don't actually go away, they still guide you on a subconscious level. You just don't "hear" them as you did when actively subvocalizing.