subtitles: show timed-fragments of the same text line
After suprisingly open the mkv with Aegisub itself, I found out that to achieve this effect, it uses a tag called alpha (like so:{\alpha&amp;HFF&amp;}) that changes the transparency of the text. 

To do this effect, you need to duplicate the same line as many times as you need (no need to cut the original transcription), then use the alpha tag to make the text invisible afterwards, like so:

N-{\alpha&amp;HFF&amp;}No! It can't be!
N-No!{\alpha&amp;HFF&amp;} It can't be!
N-No! It can't be!


To achieve this:

N-
N-No!
N-No! It can't be!


Bear in mind that putting the tag before a comma or a dot could cause some trouble with text positioning (it is supposed to be centered). Instead I recommend use this after the comma or dot.