1) A very intuitive person who just happens to be able to figure out technology just by playing around with it, usually without any prior experience with said technology. This person is able to figure out any technology, from cell phones, to car stereos, to paper shredders.
2) Never needs instructions on assembly of any new technology and always does it right.
Yogi's such a techack. He's never even used that cell phone before and he's managed to program his own ring.