How exactly does WiFi work on my EVO? Does it know to choose the best option for getting to the internet?

Here's why I ask: I have an app that syncs info between the internet and the phone. When I can't get a signal, it can't sync. Often I have crappy reception at home, one or two bars only (a whole 'nother thread), but I do have a strong wifi signal.

Being a newbie to all this, in my little world, logic assumes that the phone would automatically switch from 3or4G to wifi for the exchange of data (but not for talking). But I think I don't have it right. Can someone explain it to me?

Thanks--d