There are two ways to use a pie menu with a mouse: with or without holding down the mouse button. You can start by going "click move click", but with experience, you'll learn to go "press move release". The relaxed "click move click" way to use pie menus is to click the button (press and release without moving), move the cursor, then click the button again. The accelerated "press move release" way is to press down and hold the button, move the cursor, then release the button.
When you first pop up a pie menu, the cursor begins in the menu center, and the selection depends on the direction you move. Each pie menu "item" is shaped like a slice of pie, arranged around the cursor in different directions. The center of the pie menu is an inactive area, which doesn't select any items: so you can click once to pop up a pie menu, then click again in the center without moving, to cancel.
The further out from the menu center you move the cursor, the more precisely you control the direction, and the easier it is to select a particular item. You can move the cursor far out to the edge of the screen, for very exact control.
It doesn't matter what path you take to select a pie menu item, the only thing that counts is the direction between the first and final click. This allows you to reselect different items any time before the final click, by moving the mouse around to different slices of the pie.
If you click the button, the pop-up window will appear on the screen instantly. But if you hold the button down and move, the menu display will be pre-empted until you pause. Once you're familiar with the directions, you can press the button and move without hesitating, selecting from the menu very quickly.
You can "mouse ahead" through a pie menu, by smoothly pressing, moving, and releasing the button without hesitating, and the window will never be displayed on the screen!
Even before its window pops up, an invisible pie menu gives you instant feedback by changing the cursor shape to show how many items there are, and which item is selected.
If you want to be sure of the selection, just stop moving, and the pie menu will pop up. You can always change the selection by moving around the menu.
With experience you will be able to reliably "mouse ahead" and even select items from nested pie menus, without looking at the screen.
A pie menu can have any number of submenu levels arranged in a nested tree. Any item can pop up another pie submenu, that itself can contain items with submenus.
Clicking in an item with a submenu pops it up, centered on the cursor, on top of the previous menu. Clicking again in the center of the submenu cancels the whole tree. But clicking the other button goes back to the previous level.
When you press the other button down, the cursor moves back to the current menu center. When you release the other button in the menu center, you go back to the previous menu, positioned in the same place you were when you left it. But you can also move the cursor out of the menu center before releasing the other button, and you will stay at the current level. This is convenient if you just want to get back to the menu center and select another item.
Pie menus may be limited to a certain number of slices (like 8), so that the pie slices are wide and easy to select. If there are extra items, then they are clustered into groups of the same number outside of the pie. You can select any of the extra items by pointing directly its label.
Each group of items is arranged like a compact pie menu of the same number of items (or fewer). You can scroll the pie menu to any group by clicking in the center of the group, and the menu will center on the labels. The "Page Up" and "Page Down" keys also scroll the pie from group to group.
Pie menus support several keyboard accellerators.