Heap burning coals on his head.

The other day when I was with my connect group from church one of the girls read this scripture out to me:

If your enemy is hungry, give him food to eat;
if he is thirsty, give him water to drink.
In doing this, you will heap burning coals on his head,
and the LORD will reward you.
Prov 25:21-22

First when I heard it I was confused. Why would I want to heap burning coals on ones head? That would hurt! So I asked them to explain to me what this scripture meant and they said:

What happens if you have many burning coals in one place? It will probably turn into fire, right? Imagen that you put some of the holy fire on someones head that is not a believer, by doing good in this broken world. Wouldn't that annoy him?
What if you do it over and over again, and if other people does it as well, and a fire brakes out? In the bible they usually talk about the fire as in the Holy Spirit. What if we - through our doings - could invite the Holy Spirit upon peoples lives! At least I get inspired! I wanna see people get on fire! It starts by doing something... what does your neighbor need? Would you help him? Amen!