They go fast in hardpack conditions. They are not optimal for other types of riding. A term sometimes used for the biggest possible freeride board that a given user would choose. This is the one you take heli-boarding to Valdez Alaska. The deeper the sidecut, the more aggressively the board wants to turn.
Some boards have symmetrical sidecuts while others have progressive sidecuts.
This effects the feel of the board through a turn. This means that both tip and tail are raised from the snow, and that the board can be ridden switch. A pure twin is a board that is shaped identically on each side of it's center point, and has the same flex pattern in it's nose and tail. This is a game of nuance and will make less difference to the ride than other features. Softer noses will also help in powder, but are a little unstable when riding switch.