The jab stitch is putting the needle through the quilt (straight down with right hand if right handed) and then pushing it back up with the left hand underneath, approximately 1/4 inch away. This is personal preference on how far apart you want them. I do a lot of stippling so I follow the design that I have traced on the quilt. I use a hoop that sits on my knees so it is easy to have my left hand underneath the quilt.
I was not able to master the rocking stitch and my teacher who wins prizes for her quilts uses this jab stitch. It takes practice to get consistant but that's the same whatever you do.
I hope I explained this well enough.