It only appears when you've favourited more than one Ambience.

Go to the Gallery app, create multiple ambiences from your photos in the pulley menu on each photo.

Back out to the Gallery front page and in Ambiences, click on the ambiences you've created and make them favourites by clicking on the star under each photo.

Now on the multi-tasking view, edge swipe in from the left or right and you have the horror that is the Ambience switcher.

Now that you've seen that, go back into the Gallery and un-favourite all your ambiences and you'll not see the Ambience switcher by accident ever again.

Not always. Tweetian for instance has a pulley menu of just two items and they're both the same on the top and bottom. The reason for the bottom pulley menu is that with twitter you might have scrolled all the way down the list of tweets and the pulley is now off the top of the list. Tweetian has a permanent toolbar with icons so a bottom pulley menu is always available no matter where in the list you are. This is the main problem with pulley menus - they're attached to the content usually, not the chrome if indeed there is any chrome at all.