It is a musical term defined as a gradual increase in loudness, true, but it is a tendency, a propensity, I’ve noticed of late among Korean actors. When they are angry, they start out soft, then shout, the intensity increasing gradually. The first one I noticed using this technique was the Prime Minister in The Prime Minister and I. And a few others since.
Usually, the older actors don’t employ the technique, thank goodness, which makes me think it’s a new one the K actors have been resorting to.
I’ll take note of which actor does this in which series, but perhaps, you’ve noted the same yourself? Please identify them in the comments section below, if you are so inclined. I want my observation to be validated, hahaha.
Here’s another mannerism I’ve noticed among K drama leading men, in pictures: