Standing for "slip, slip, knit," this technique reduces the stitch count by one while slanting the resulting stitch to the left. The knitter slips two stitches knitwise one at a time, inserts the left needle into the fronts of both, and knits them together. It is frequently paired with a right-leaning decrease to shape garments symmetrically, especially around armholes and necklines.
Heard on the job
I usually swap out the SKPO for an SSK on the left side of the raglan shaping because it lays much flatter.
Insider tip
Slipping the stitches knitwise twists them just enough so they lay flat when knitted together, avoiding the bulky bump that other left-leaning decreases can leave.