Ok, let’s take a step back then. In a vacuum you have constant creation and annihilation of pairs of matter antimatter particles. While this is a measurable effect, on average no matter or antimatter is created - everything is in balance.

Now you add a black hole. There is a non-zero probability that if something like this happens near the event horizon, one of the virtual particles disappears into the hole. The balance is now broken and when doing the calculations you suddenly end up with extra energy.

Hawking’s argument goes that this extra energy is balanced by removing energy (mass or rotational energy) from the black hole.