The XP1900 cannot be unlocked by joining the bridges with a pencil. I believe it was the older Tbirds that you could do that with.
For the xp1900 you have to fill the pits in between the bridges with some kind of, well, filler. When I did it I used super glue to fill the pits and then a Rear Windshield Defogger repair kit to join the bridges. DJ-Quack wrote a nice little guide on unlocking paliminos. You can find it
HERE. It will walk you through the steps.
And as to what unlocking does... well, it unlocks the multiplier on the chip allowing you to drop the Multi and raise the bus speed. I think that about covers all the bases.

Have Fun