when we select that tool
»Tool« has a specific meaning in Photoshop, »High Pass« is a »Filter«.
For an explanation of the connection to Gaussian Blur check out:
High Pass can not only be used for sharpening but can also helpful for extracting patterns from images.
Edit: I should have clarified that I meant Patterns intended for use as Linear Light Pattern Overlays to allow painting on a Layer while maintaing a sctructure on the paint strokes to better fit in existing images.
Jeff Schewe, in his new book “The Digital Negative” (Peachpit Press) describes a use of the High Pass filter to adjust midtone contrast -- bring up midtone contrast details -- somewhat like the Clarity adjustment in camera raw, but “you have a lot more control over the parameters.” He applies a High Pass to a layer copy of an image with blend mode set to Overlay and using Blend If options to constrain effect to the middle range of levels in the image. (pg 210-212)
The high pass filter is a tool for finding edges...from that, you can adjust the edge width to increase or decrease the radius of the edges. Everything that is not an "edge" turns into a middle grays. Depending on the blending, this give an opportunity to modify edge or non-edge values. Overlay lightens the lights while darkening the darks and does nothing at 50% grays In the case of high pass, this means the light side of the edges gets lighter and the dark edges darker. This can have a sharpening effect similar to USM without the threashold control.
If the HP radius is small, the effect is sharpening...if the radius is larger, it's more of a tone /contrast control. If you constrain the effects using Blend If adjustments, you can impact midtone contrast as mentioned by Robert.