Whoa, jchensor must have received a visit from the ghost of GameFAQs past because he’s written a new technical guide for Super Street Fighter IV! Here’s the link and a brief quote:
What is Plinking?
Plinking is a technique first written about here on SRK by KIRBYSIM. It technically stands for “P-Linking,” which itself stands for “Priority-Linking.” It’s a technique utilized by many players to help land their 1-frame and 2-frame links more consistently in battle by taking advantage of a trick that allows you to repeat a button input for 2 frames in a row.
The term “Priority Linking” is used because of the nature of how Plinking works: the input you want repeated can be controlled knowing the “priority” of the buttons in Street Fighter IV.