A split stitch creates a new stitch through another one to prevent (lace) holes when you increase in a knit (not at the edge).
It is a transfer but the transferring needle immediately receives yarn from a feeder too. When you transfer you move a loop from one needle (delivering needle) to another one (receiving needle). Standard transferring leaves the delivering needle without a loop, creating a hole in the fabric. By using a split stitch, you fill that needle with yarn immediately after the transfer, avoiding the hole in the fabric.