Bruising shows how much... um... "force damage"(?) a joint is being subjected to. That also includes if you move it in a way that increases the force being applied - eg someone grabs an arm and you pull it back increasing the force.
But yeah, bruising occurs under all forces that I guess you could say cause damage. Standard Toris in standard gravity don't move in a way that'll cause bruising just from using a joint, but if you prod the settings for body parts in a mod you can make a joint bruise just from moving it yourself.
I'm pretty bad at explaining it, but in short bruising is an indicator of how much force is being applied to a joint at a given time; that includes impacts (if you read the ODE engine wiki you can master how impacts work, I cba failing at explaining that too), moving against an opposing force (eg lifting yourself up in stupidly high gravity mods, or being pushed/pulled) so on so forth.
As I said though, you have to get a feel for it and some colours are almost useless for judging it. Just don't use red forces or incredibly dark colours (Void, Hunter & Alpha are the worst offenders) and it's pretty easy to learn.