WW, you tape the pins to trick the lens into autofocusing. Otherwise the autofocus shuts down when you fit a teleconverter to a lens that is f5.6 or slower (now you wish you hadn't asked
)
You can use clear tape.
I believe the AF doesn't shut down on pro bodies, so I think Dai is right. But pro bodies have no crop factor, or 1.3 crop factor, so need a TC more than consumer bodies do!
If you read the various photography forums, it seems the success of the pin-taping trick really varies from individual to individual. It doesn't work well on the 400 f5.6 (AF really hunts and oscillates) although I recently read of someone who tried it on 2 different 400 f5.6 lenses, and it worked OK on one but not the other
More success seems to be had with the 100-400 although it seems difficult to explain why this would be. Others have reported different success with different camera bodies
Matt