Keeping it churned up ensures that it mixes and dissolves well, also serves to help keep it aeriated which helps with the ph and oxygen. I use a maxi jet powerhead that will allow me to attatch an airline tube to send jets of bubbles into the water. All this gets mixed seperately from the main tank.
Before doing the water change you make sure that the temp of the water change water matches the temp of the main tank as well as the salinity. This will help to make sure that you are not making to drastic of a change in the water chemistry that will not shock your fish or corals.
Never add raw salt directly to the main tank, as you know salt burns and would irritate your fish and corals.
Mixing it for at least 24 hour allows it to stablize some.
Got it?
Thomas