You can thread the belt throught the Y-hook but many belts can be flexed enough to just put the Y-hook on the belt. A "tuckable" holster allows the shirt to be tucked in normally and the gun is completely out of sight. It would be against your skin unless you wear a t-shirt or something similar.
I often wear an Ultra Deep Cover completely tucked and nothing else underneath and it doesn't bother me but that would be an individual matter depending on the gun and the carrier.