First I'll let you know that I'm a newby with aneros. I just had my third session today. But I do have 18 months experience doing a Tantra like exercise 30 minutes every day. My focus in those meditations was the PC muscle, but I think my idea would apply to the anal muscles as well.
When I started I had very little stamina or strength in contracting my PC muscle. Over the months I gained strength and was able to contract more strongly and hold those contractions for a long time. But strength is only part of the control. When I started I pretty much had two or three settings for my contractions – 1. no contraction, 2. medium contraction, 3. full contraction. Over the weeks and months I specifically focused on moving smoothly from relaxed to contracted. As I might grip a soft rubber ball very softly and gradually, slowly squeeze tighter.
As I paid attention, I sensed there is very high resolution in these muscles. By high resolution I mean very small steps from one level to the next. And each of those steps has something to offer. While I think it's good to get stronger so that you can extend the extremes, I think it's probably more fruitful to carefully examine each of finest differences is contraction.
The other thing that I've found is that the whole idea of contracting muscles soon goes away. In squeezing a ball, we don't think "contract muscles". We just squeeze the ball. Eventually, those sensations that come from a somewhat mechanical approach will be directly accessible. You can just feel it, you don't have tell yourself how to anymore. Boy this is exciting.