In
wckf92's defense chasing hands is definitely a thing, and usually a bad thing to do, but how one defines it might differ from one group the the next.
I've seen some people argue that if you do anything other than attempt to hit a person (often you will hear "attack the centre") even if that anything includes dealing with an oncoming attack, a punch to the head perhaps, then you are chasing hands. I can see the rationale behind such an argument but I wouldn't agree with it. For a start, in both styles of wing chun I learned attacking the wrist of an incoming attacking arm is an entirely legitimate target and more often than not done in conjunction with an attack to elsewhere on the body. Moreover, you don't need to chase anything if some guy's fist is chasing your face.
What I would characterise as chasing hands would be, lets say someone has thrown a punch, you have caught the punch with, maybe, tan sao. The guy punching is extended and so to redeploy that punch (lets assume he is one armed for the sake of the illustration) he needs to retrieve it and relaunch a new punch.
Now, were I to follow that punch back and take the first opportunity to hit the guy when I had the line, I am following "force comes deflect, force retreats follow, if there's no force rush in and attack". If, however, I forgo hitting the guy once a clear line presented itself in order to stick to the guy's arm then I'm chasing hands. There may be reasons why one might want to do that, but in general its something to avoid from my point of view.