The HPF button: Possibly the most underused button on the desk
The High Pass Filter button, also known as a Low Cut button, is a common feature on many desks of a larger size, both analog and digital (although in the digital realm you have much more control). It does exactly what it says - it lets the high frequencies pass through / it cuts the low frequencies (the bass).
On a nice analog desk, like the Allen and Heath GL2400, it's right beneath the gain knob (see image), and this is where it is typically found on every channel.
You will see it has what looks like almost a square-root symbol with a frequency labelled underneath: this is where the desk will roll off, or in simpler terms, 'cut' any frequencies beneath this frequency. Some desks it isn't labelled so you'll need to check the specs of the desk in the manual.
You will notice on the stereo input channels that there is no HPF (on the GL2400) and as such I've highlighted the low frequency EQ dials. You can use these at a -15dB cut to try and emulate the HPF but it won't be quite the same, although you won't typically be having an instrument or source that will need that much of a cut on these channels unless you are running out of inputs.
The HPF is great to use when you think your mix is beginning to sound muddy. I use it on every channel besides the bass and kick drum in a full band environment, and in a smaller band where there isn't much coverage of the lows I won't use it on say the piano or maybe even acoustic guitar and boost their lows for a bit of punch.
The reason I have it on every channel? Well if you check out the frequencies of pretty much most instruments you will see not much lies beneath 100Hz, and if you can save your amp a bit of work in the lows then they will be a lot cleaner coming out the speakers as there will only be a solid bass line (if your bass player is any good...) and a defined kick drum for the punchy lows. There may be a bit of male vocal at around 90Hz but that's not very common, and usually all you'll get is mic handling noise or footsteps on the stage, so best to cut it.
The best way to remember it: this lets the highs pass through a filter. It helps avoid low end feedback (particularly if you have a lot of power in your subs).
If you want to get more advanced with this feature, on some desks you can select the frequency at which it rolls off. For example, cymbals will need to be cut way higher up than 100Hz (particularly if you're doing a full kit mic, which is a great shout) as cymbals aren't designed to create many mid- or bass-frequencies. The more you can cut than boost, the better. Conversely, an acoustic guitar has a bit of 'body' at 80Hz so it may not be the best solution to use the HPF there if you're in an acoustic environment (unless you can drop it down to below 80Hz) - if you have a bassist, then it's fine to cut this.
More on the EQ coming soon folks, that's all for now...