I haven't seen one in real life, but as I understand it the nut doesn't have grooves, instead there are 4 pins sticking up. The string pegs are angled in such a way that the string rests (is pressed) against a pin. When there are no grooves you don't have to worry about having the exact right width of the groove for the string diameter. With a traditional grooved nut the width of the groove should match the string, not too narrow (makes tuning difficult - the string would "stick" and jump), not too wide (creates buzzing). So if you want to use a different gauge you may have to change the nut. By using those "pinned nuts" you avoid all of those problems.
I'm not sure what happens if you try to bend the string in the direction away from the pin though.. maybe there's a detail to that which I'm not aware of.
All of the above is only what I could figure out without actually having held one of those ukes in my hand, so take it with a pinch of salt please.