Why does the downward velocity of the parachutist decrease after the chute is opened?

There are 2 types of parachute - the round parachute and the more modern wing parachute. The round parachute is a decelerator and works on drag only through the atmosphere. However, with the wing shaped 'square' parachute, they act with an additional force of lift. Another name for this type of parachute is the Ram Air Parachute.

Whilst few of them ever generate enough lift to gain altitude, they do create masses of forward motion which gives them better range to land back on the dropzone. You also get better, safer landings which are easier on the knees! Most dropzones these days only use squares.

I would recommend a tandem jump for your first one which uses this more modern type of parachute.

A parachute is deployed using a smaller drogue pilot chute which drags the main parachute off the container. The container is a combined harness which holds the packed parachute on your back. As the parachute inflates, it will slow you down from 120mph to around 10mph average. The final stage is landing, where the skydiver pulls down on both steering toggles to further slow down the decent rate for a tip-toe soft landing.