The source neutral wire on the receptacle is removed and spliced to the white running to the switch, and to a pigtail back to the receptacle neutral. At the switch, the neutral wire is needed to power some dimmer switches and is now required in most switch boxes.
This circuit diagram shows the wiring for a new receptacle outlet connected at a light fixture. The source at the fixture is always hot and a switch loop controls the light. New 2-wire cable runs from the fixture box to the new receptacle and the source hot, neutral, and ground are splice to it along with a pigtail back to the fixture circuit. With this arrangement the new receptacle is always hot.
The white wire on the switch loop is used to carry current from the source to the switch and it is marked with black tape or paint to label it as hot. The black on the switch loop runs from the top switch terminal to the top half of the receptacle. With this arrangement the top half of the duplex is controlled with the switch and the bottom half is always hot.
This page contains several diagrams for wiring a switch to control one or more receptacle outlets including a split receptacle and multiple outlets wired together.