The high pass filter for Affinity Photo can be found in the sharpen menu (beneath the Affinity Photo filter menu).
It is a powerful tool for creating all kinds of sharpening the edge of images and embossed edge-like effects and much more. You can also use the effect with images as well as other resources such as fonts and brush strokes and patterns and more. The effect is a tool to generate an image or layer than can be then combined with the original image to hopefully improve or at least, fit the requirement for your artistic vision for the image or text or shape etc.
It has a few options (two) includes radius and monochrome. The easiest way to use it is create a duplicate layer of the original image and use blending modes. There is also a live layer and that comes with the same options and blending modes and opacity. It is great for ghostly effects. It should be noted that the effect is in the Photo persona. Go to the Filter menu and Sharpen and High pass and then Set radius etc and click Apply
There are not many options available, just two and that is the radius of the effect and the monochrome option (which is a bit of an odd one as you can always change the color to a black and white etc / monochrome etc by using it with an adjustment layer. The radius at 0 shows hardly anything beyond a very gray image but as you increase the radius setting to change the edge sharpening, you will see more and more of the original image and more of a glow around the image though you will never remove the grayness. At the very highest setting, the effect gives a slightly surreal gray glow to the image and very little of the fine edge sharpening effect. You can also use the mouse to modify the setting of the radius by dragging across the image
Trying the effect with a fairly blurred image is a bit of a waste of time. It works best with images that are reasonably sharp to begin with, it will not work miracles and retrieve a smudged image.
I generally go for a duplicate of the image before I start to use an effect as I can always then just delete the result if I am not happy with it but it gives me an option to blend the result with the original and that may have even better results than just the single application. To duplicate, go to the layer menu and duplicate and then use the effect. Go to the layer menu and change the blending mode as required. The effect does not block you in changing the blending mode so you can alter that as you set the radius. You can then use the effect for some interesting glow effects say in color burn blending mode
As with all others, you can combine with others such as deform, I really don't see them as just single shot tools. You can now also record all the settings into a macro and store the macro away for future use.
Instead of using a layer and blending modes, you can also use the effect as a live layer via the layer menu and the layer has the blending mode and opacity built in and it is live so the result can be changed at any point as well as deleted. In some tasks, the live effect proves a very useful alternative and especially combined with adjustment layers as it does mean instead of creating a gray look. You can also add into the mix any of the other (albeit) small number of sharpen tools as well as blurs etc all as live effects which can be changed and tweaked depending on the project. If you wish to apply it to all layers, deselect all the layers. If you wish to apply it to a particular layer, select that layer. Goto the Layer menu and select the new live layer and effect and set radius and set monochrome or not and set blending mode and the opacity. Close the panel or leave open
You can use it with the pattern layers, it is best to set the mirror option (top left) to on when using these layers.
It can be used for some very subtle edge sharpening changes and final touches for an image. It can also be used for a whole lot more such glows, ghostly effects and more
It can be used to create all kinds of amazing colorful designs by using it with the channels panel. Channels means you can apply the effect in the red channel with one set of values and then go to the green channel and use the effect with a different set of values or perhaps combine it with others such as Gaussian blurs and others and then finally the blue channel. You can then return to RGB and have a super amazing design from your effect.