How does watering help taproot dig deeper?

[ammoun]

Every time I water mango seedlings I get the same thought. So when you water a tree it seems to me quite impossible for the moisture to reach the bottom end of the taproot.
In my sandy soil for example, it gets really tough to dig farther as you get deeper unless you add some water to the bottom, so how can I help the taproot when watering if the added water doesn't get as deep as the end of the taproot?

I hope I am not sounding very confusing, let me know if what I'm asking does not make much sense.