The base of a NPN transistor opens when you "push" it up.
The base of a PNP transistor opens when you "pull" it down.
Remember the two diodes i told of ?
Actually it's more like one and a half diode.
The N stands for Negative layer , the P for Positive layer.
The diode part means that current is stopped when it reaches the center layer.
But as soon as you start manipulating that center layer by applying a current there, the diode function is altered, and the outer layers will bridged by the now conducting center layer.
That's how a transistor functions in a nutshell, there's much more about it.