I use a form of concentration practice called zhine that is used in Tibetan dream yoga. It is similar to kasina, in that it involves sitting and staring at a little coloured disc. You're supposed to keep your eyes from blinking and gaze constantly at the disc. If the eyes move it's very noticeable! But, you do learn to deal with it effectively.
What I have noticed is that the movements of my eyes correspond with movements in my mind. As I become more concentrated, my eyes move less. The eyes are the windows to the soul, after all. (Note: the eyes are constantly moving, but in very small, generally imperceptible ways. Just to be clear, I'm referring to the movements that we would actually call movements in general life.)
I also find that when "on the precipice" before a deeper level of concentration, extra mental energy occurs. I think this is because the energy being wasted in unconcentrated mental activity becomes available as that activity ends. It feels good and can manifest as joy, and/or can be overwhelming and distracting. If I make an effort then these changes are not distracting and actual form the basis for entering the next level. Greater mental energy allied with good focus are, naturally, what you'd require for a deeper level, so it would seem to follow.
The main thing I try to remember is that it is not the goal to become concentrated, it is the goal to practise concentration. Concentration is a by-product of concentration practice. Just as stronger muscles are by-product of physical exercise. What's important in both is to focus on a) doing it, and b) following the correct technique.
Nowhere have I ever seen a practice include the directive "and when you lose concentration, become annoyed with yourself". Distraction is normal, all you need to do is to be aware you've lost some focus, and then refocus.
Hope that helps.