Spike stitches (Spst) are a variation/extension of the sc. While a sc is worked into the previous round, spike stitches are worked into a stitch 2 or more rounds below.
How to work a Spike stitch
Insert the hook into the indicated stitch, yarn over and pull the loop up to the level of the current round, finish as a normal sc encompassing everything in between.
Spike stitches can be straight down or angular. They are worked next to an already existing stitch into the same top loops.