One of the attributes of 劲 is the vector 'impulse', as measured by: IMPULSE = FORCE x TIME
短劲 is when the time of force application is short, like a car hitting another car. It is about releasing energy in a burst.
长劲 is when the time of force application is relatively longer, like a train engine pulling its carriages. It is about releasing energy over longer time period.
寸劲 is another concept. It refers to the distance of acceleration of the fist (or other body parts), which can be as short as an inch. When the fist hits the opponent, it is still possible to apply either 长劲 or 短劲.
寸劲 is also deceptive in terms, because even at the point of contact, the other body parts (hip, waist, spine, shoulder, etc.) can still move, resulting in bigger energy transfer.
taichicat says it is easier to 化 短劲 than 长劲, it is because he hasn't met the real taichi super cat or they are too nice to use it on him because it is dangerous.