It sounds like he's talking about "chicken Picking", where you alternate between using the the pick and the middle finger to attack the strings. It produces a twangy sound favored by Country and Blues guitarists, and Mark Knopfler.

If your finger got sore thats because you over did it for the 1st time. You have to start slow and build up stamina. Imagine a bass player learning to slap and pop strings for the 1st time and then spending 2 hrs doing so, his right hand is going hurt for the next few days.

Do not use your nails for this technique, they will produce an edgy sound, plus steel strings will tear up your nails real quick. You want the twangy effect produced by using your finger.