The Up and Under Move is counter to the Power Move. A Counter move means having an option if the defense takes something away. A team may take away your first option on a play, so you need a counter. What happens once they take that away? You go with a second option or a counter. If they take the Power Move away, you have the Up and Under as a counter.
Instead of making a step toward the basket the player will use the same foot (closest to the baseline) and step around to the lane Pivoting on top foot. Now facing the basket and sell the shot fake. After shot fake step across the body and will use a two-hand dribble to get close to the basket and go up for the shot.
Your teaching points are;
1. Stay low on the Pivot and turn all the way around to face the basket. Don't fake the sideline. Turn all the way because this sets up the move.

2. Sell the Shot fake. Get the defender off their feet and off balance by showing the ball.

3. Step across the body and get an angle by the defender.
I want to think of how I am Getting by a defender any time I work on a move. Long step to get closer to the basket and sealing off the defense with the rear and hips.

Having another option in addition to the Power Move makes you more effective in the Post.

So if the defense completely takes the baseline and Power Move away they can't shut you down. You have another way to Score for your Team.
The Up and Under is valuable because it puts you close to the basket for the high percentage shot, and with a solid fake gets defender off balance so you can draw the foul or have a clear path to the basket