It's a skill. A learned skill through diligent stroke counting practice. Much the same way you try and lower your stroke counts you can teach your nervous system to add strokes at will; along with complete awareness of your balance and streamlining (slipperyness) and how to adjust / refine those.
It's quite encouraging when not only do you hit a consistent # of strokes per length but you do it because you planned to. Then changing the # per length at will is another level indeed. Terry has been at it for years and continues to give his nervous system the necessary attention it needs in order for him to retain his level of skill.
I'm having a measure of success with it and am sure Suzanne (and many others) is too.
I'm having fun working on getting to that next level. "To become effortless takes great work".