There seems to be a lot of things that come together to make it work, but it's basically sucking not pumping. The term to google is Transpiration.

It's a bit like a siphon effect with water evaporating from the leaves creating low pressure internally which draws more water up, and the reason it's able to pull a whole column of water up is because water molecules stick together to some extent via hydrogen bonds.

Given that evaporation is what is driving it, I wonder how that works with evergreens with low evaporation - I guess it's basically a replacement system, so you only need to pull what you evaporate.