I've had the chance to do some Worm parcel and serving volunteering for a nonprofit building a wooden boat. It's SOOOO labor intensive, but pretty darn satisfying.
According to the lead rigger, when tightening the serving, you need to apply enough force to "squeeze the tar out of it". The tar impregnated line kinda oozes a layer of tar onto the surface. Hence the expression ___ the tar out of something. At least according to Jim Nelson, but he is a writer of pirate fiction novels, so maybe not 100%reliable