Explanation of changing particles within a blackhole

[only_huce]

I am currently reading A Brief History of Time and after reading chapter 7 there are still concepts which I do not understand.

Hawking explains that particles go into black holes while antiparticles are thrown outwards into infinity. How and why does this happen?

He also goes on to explain that when a particle goes into a black hole (ie. a positive particle) the black hole can change it into a negative particle. How does this happen specifically?

 

[chroot]

If a virtual particle pair is created near the black hole's event horizon, either of the two particles may fall into the black hole, just by chance. The other may escape to infinity if its initial trajectory was just right.

Since before this event you have just a black hole by itself, and afterwards you have a black hole + a particle, it is exactly as if the black hole emitted the particle.

- Warren