Tics are basically an irresistable urge to do something, sometimes people can stop them, or deprive themselves of following through on them, but when they do this, it causes them much anxiety and usually takes all of their focus and energy to 'ignore' or supress the urge. Stimming (imo) is voluntary, we do it because it feels good. Sometimes we do it because it helps us focus. However, I think if anyone did a certain stim over and over enough times in their life, it may just become a habit (like my cheek chewing). When it's a habit, it becomes more unconcious of an action, so it may seem like an irresistable urge, which is hard to ignore.
Like right now, wiggling my foot while I type is a stim.
I feel bad for people with tics. They have two choices, indulge, scream "F*CK!" (or whatever other tic it may be, possibly repeated eye blinking) ...and look/sound like a freak, or supress the urge, and loose all focus until they can find an acceptable outlet.