I have not afforded this one yet, but it makes sense to me to make two cuts and then press it open (for half square triangles). It comes in a several versions. For instance one is for whole and another is for 1/2 inch increments, I believe. I copied this from the Missouri Star website to explain the Clearly Perfect Slotted Trimmer. It looked interesting to me when I saw it demonstrated at Quilt Festival in Houston.
The Clearly Perfect Slotted Trimmers are an easy, efficient, and accurate way to square up half-square triangles. Simply align the seam of an unpressed half-square triangle with dashed lines and trim all four sides. The edge slots allow you to trim off the dog ears at the same time! It can also be used for squaring up hourglass quilt blocks, strip piecing half-square triangles, strip-tube quilt lock constructions, and fussy cutting. The B2 trims full inch sizes from 1" to 10".