Although she may appear to be sitting on the eggs, the bird will keep the breast feathers closed so incubation doesn't actually start yet. This dries out the nest and will enable her to lay most of her eggs before she bares her broodpatch when sitting on the eggs and actual incubation starts. While birds lay eggs some of the lower chest feathers will fall out and a patch develops that is gorged with bloodvessels which enables to skin to transfer the bird's warmth to the eggs. The chicks will thus all hatch pretty much on the same day.
As Chaffinch says, the hen may leave the eggs for a very short period to void and have a quick stretch. I am pretty sure that swallows will not feed when incubating though, it will take too long to catch enough insects to sustain her. The male will need to feed her and subsequently the young chicks.