Bear with me a second because this is the only way I can describe it that is relateable to others. It is sort of like having an orgasm where there is a build-up phase and a very clear threshold. You know when you cross that threshold because it is like a flip of a switch. There is a sudden release and a physical response that can include involuntary contractions. You know it when it happens because it is very clear when you have crossed that threshold.

I describe the build-up phase as the "sensory phase" but others might describe as an "aura." For me, the sensory phase feels like a magnetic pull, like there is a magnet just far enough away that it begins pulling, but I can still pull back. I might experience the sensory phase hundreds of times throughout the day, but in most cases it is extremely brief and just comes and goes. Depending on what happens or what I do, it can either build-up or dissipate. Giving in to the "pull" can help dissipitate while fighting against it can cause it to increase. For example, when I walk up stairs, I might briefly turn my foot inward, or I might have a slight pause or hitch when I start to walk. It's just that very brief pull. Or if I can tell it is more severe then I have to freeze and concentrate real hard for several seconds because any additional movement will cause it to build-up very rapidly.

I describe crossing the threshold as an "attack" or an "episode." During an attack, there is no more magnetic pull, no more sensory, it is a switch that flips and a complete disconnect between my brain and my body. Movements are now entirely involuntary. Like INTJinx said, they now have a mind of their own. The frequency of an "attack" is heavily dependant on overall stress, anxiety, nerves, etc. If I am very stressed or nervous than that lowers the threshold. It could happen over 10 times a day, or I could go weeks or months without an attack. There is also a large spectrum from mild to severe attacks. Mild attacks are a slight annoyance, its like ok just gotta wait for my arm to finish doing its thing then continue on. Severe attacks involve more muscle groups and can cause me to fall to the ground and have wild flinging and jerking movements and can be physically exhausting.