Highpass is a really old one! It’s a simple one-pole Airwindows interleaved highpass filter, but it has a weird control (because of course it does). That control is loose/tight, and it’s one of the most spectacular secret weapons for bass/subs mastering ever.
In the middle, it returns a very normal sort of highpass, not too steep in slope.
Set to Tight (all the way to the right) it acts like high transients are the goal in life. Lows around the cutoff are held back, like the higher frequencies are directing them. This is done by input level modulating the cutoff, so louder means ‘less lows’. Lows only come through if they’re gentle. It really reins things in, softening the lows by stepping on them harder when they’re loud.
Set to Loose (all the way to the left) it’s the opposite. Things are restricted, but when the lows kick in they slam. It exaggerates the subs a bit, at least makes them seem to kick in extra hard. This setting will also tighten up sustainy lows on kicks (which is ironic, since it’s labeled Loose).
The way to think of it is, the bigness of the swing of the bass transient. Tight means the bass kicks are tight and higher in frequency for whatever the cutoff is. Loose means the bass kicks go deep and wild, seeming to get more carried away, while the body of the music feels more bass-restricted. And of course you can adjust that more finely with the control once you’re familiar with what it’s doing to the subs.
Highpass is a good tool for handling bass. Not all highpasses are created equal.