I don't know if this is the way those are done, but I would handle those like...
Code:
{\pos(320,450)\1a&Hdd&\bord4}text
{\pos(320,450)\1a&Hdd&\bord3}text
{\pos(320,450)\1a&Hdd&\bord2}text
{\pos(320,450)\1a&Hdd&\bord1}text
(the time codes omitted)
With \pos, you don't have to worry about the overlapped sub being placed on top of each other on the screen.
If you use ASS layer field effectively, then you can eliminate \pos from the code too.