Typically, pie menus support both "click up, point, click down" browsing and "press down, drag, release" gestures.

Pie menus can support "click up" tracking compatibly with "press/drag/release" tracking, even allowing users to switch between both modes when browsing submenus.

This is important for supporting both novice and expert users, learnability, rehearsal and muscle memory.

You can divide the pie menu learning curve into three stages:

1) The first stage is how a new user typically uses it when they don't know the directions: click it up, look at the menu items, decide what you want, point at it, click it down. It requires lots of time, visual feedback, and mental attention, but it transparently trains you for later stages.

2) Then there's an intermediate state where the user knows the direction, presses down, moves, then looks at the screen, waits for the menu to draw and give some feedback that they've selected the right item, then finally releases the button to conform, or moves to correct the selection. It requires much less time, but still requires some visual attention and waiting for feedback before confirming the selection. But it reinforces previous learning, and builds muscle memory and confidence.

3) The final stage is when the user remembers the direction both mentally in "neural memory" and physically in "muscle memory", so they can just quickly flick in the correct direction and go on to their next task immediately, without looking at the menu or waiting for feedback. This requires the least amount of time and is the most fluid and satisfying way to use pie menus.

Carefully designed pie menus can seamlessly support all three stages, and both modes of interaction (click up, or press/drag/release). That enables users to smoothly escalate from one stage to the next.

There are several properties that dovetail together to make pie menus easier to learn:

1) Pie menus bridge between menus and gestures with a smooth learning curve.

Features that support expert users shouldn't get in the way of novice users: If you don't use pie menus very often, you can still use them easily without spending time or memory to learn your way around them. But using them in "novice mode" is rehearsal for using them in "expert mode", while they aren't any harder for novices to use because of that.

Features that support inexperienced users and learning, like fancy graphics to present menu on the screen, animate theatrical feedback and help illustrate and explain the selection, should not get in the way of the more efficient ways of using them as gestures.

To contrast: Drop down menus that display keyboard shortcuts suffer from increased size and cluttering, and using drop down menus with the mouse is not rehearsal for using drop down menus via keyboard shortcuts.

2) Pie menus are "self revealing": they show users the available options and their corresponding direction, so they are easy for new users to discover, use and learn.

To contrast: Normal invisible gesture recognition is not "self revealing", since they usually do not have a way for the user to discover what gestures are available and how to make them. Some gesture recognition systems have "help dialogs" that show and explain each available gesture, but stopping what you're doing to look at a help dialog to discover and learn the gestures is not the same thing as just using the gestures to get your work done, learning them while you're using them, without spending extra time on training.

3) Pie menus seamlessly support rehearsal. Stage 1 gestures are rehearsal for stage 2 gestures are rehearsal for stage 3 gestures.

Clicking up menus at stage 1 is rehearsal for stage 2, because it shows you the options and their directions, and lets your leisurely browse the menu at your own pace without stressfully holding down the mouse button.

At stage 1, it can incrementally reveal more information about the menu and items, while previewing and explaining the effects of the selection. Once you mentally remember the directions of the items you want, you smoothly advance to stage 2 without even noticing it.

Gesturing into menus and waiting for confirmation at stage 2 is rehearsal for stage 3, once you have enough physical muscle memory and confidence to quickly use gestures without looking at the screen or waiting for feedback.

Showing live in-world feedback, like previewing the effects of the current selection in real time during menu tracking, is an effective form of direct transparent feedback that reduces the need for indirect cluttered feedback with words and icons. This works well when using the distance as a parameter to the selection, so you can "pull out" sub-items (like colors) or linear parameters (like font size), and release the button when you're happy with what you see.

Both novice and expert users benefit from that kind of "direct manipulation" feedback, since it's better to keep your attention focused on the thing you're interacting with, not the menu.

Pie menus can be carefully designed to make learning the item directions easier, by exploiting the natural directional, opposite, orthogonal, linear relationships between items. For example: twelve items for a clock face of hours, astrological signs, months, etc; or eight items for compass directions, orthogonal pairs of opposite pairs that can be combined along diagonals (like good/evil, lawful/chaotic), etc.

A humaine user interface will explain why any menu items are disabled, and make it easy for you to do whatever is required to enabled them, instead of just graying them out and not telling you what's wrong when you try to select them.

Here's a demo of pie menus for Unity3D, that shows some of these ideas:
http://www.youtube.com/watch?v=sMN1LQ7qx9g