ChuckT, have you tried to do pivot slips yet? If you've tried, were you successful, or did you find them difficult?
Getting the pivot slips to work (you travel straight down the fall line at constant speed, not traveling right nor left but straight down, not falling on your face either) is very tricky.
To do them, you face down the hill with your hips and shoulders, while you rotate your femurs in your hip sockets. Learning to do this helps people uncouple the upper body from the lower body. It's worth learning to do, because rotating those legs independently of the upper body will help you maintain your balance as you ski. If you can't face down the hill with your upper body as your legs turn left and right, you will travel left and right, and the pivot slip fails. The drill tells you immediately if you are moving your legs independently of your upper body.
However, just doing that won't get you an effective pivot slip. There are a bunch of other things you have to do or your pivot slips will not work.
You have to stay balanced in the right spot above your skis. If you have your COM behind and uphill of your skis, you'll travel left and right ... another reason for a failed pivot slip. You have to keep your COM moving down the hill along with your skis rather than doing a rocking horse thing with your upper body, alternately leaning back into the hill and then lunging forward over the skis. If you do that your travel won't be smooth and you won't be able to maintain a constant speed ... failed pivot slip. Maintaining a smooth movement of the COM down the hill over the skis is important to skiing in general. It's worth learning. Trying pivot slips will tell you immediately if you are doing it successfully or not.
With pivot slips you have to manage the movement of that inside ski so it doesn't get hooked up and in the way, blocking the general pivoting of both feet. If the inside ski gets in the way, you travel right or left ... failed pivot slip. Learning to manage that inside ski is very important for all skiing; it works the same way in pivot slips as it does in other turns. Pivot slips will inform you if you've got that down or not.
For a successful pivot slip, you need to be able to maintain just enough edge angle between the skis and the snow to provide enough grip so you slip down the hill at constant speed without making a face plant. When you can do this, you are successfully making fine manipulations of your edge angles. That's worth a million smiles when skiing. Pivot slips will let you know if you are in charge, or if you are sloppy with manipulating your edge angles.
Being able to release the skis and pivot them in sych with each other without any jerky moves is a fine motor skill you can take to all your turns on the hill. If you can't do that in a pivot slip, you travel left or right ... failed pivot slip! When you can do it, voila you have successful pivot slips going straight down the hill. Being able to make a clean release is essential if you want subtle control over your skiing at any speed and on any terrain. Learn it while making pivot slips, and you've got it in your tool bag.
Also, just for fun, if you want to do flat 360s then pivot slips get you 1/4 of the way around.