I was tightening the large nut that compresses the wedge-shaped rubber washer underneath the lavatory-bowl and the chrome flange that covers the hole in the bowl started turning. I couldn't reach it with my free hand and wondered if the HOLES in the tailpiece are for some kind of tool. Thanks, Ed

Not sure if your terms are right on with a plumbers terms. Are you tightening the toilet tank to the toilet?

If you have a vessel sink the holes vent the sink,(see image).
As for a handy tool to work under cabinets we use a lavatory or basin wrench.(seem image) which may be bought at your local big box store. Good luck, Tom

If I understand the question correctly you are trying to to tighten the nut under the lav drain that forces the rubber gasket up under the bowl and the chrome strainer on the top of lav keeps turning with the nut. Just simply stick a screw driver from the top into holes on drain to hold back on until it is tight and then remove screw driver. The holes are for the overflow but can be used to keep the drain from turning while tightening lave nut. Use plumbers putty under chrome flange and be careful not to scratch it with the screw driver.