Hold down the Alt/Option key and click on the keyframe.
I am using PC and I have to hold the Ctrl and click the keyframe.
Thank you Rich and Mylenium.
Thanks All...still helpful in 2016!
Roland has it right. It's CNTRL+click. Thanks!
To expand on Rick's suggestion: Holding down Alt toggles between linear and Auto-Bezier keyframes. This also works on paths in the composition window as well as masks, as it effectively invokes the anchor point conversion tool which is otherwise available as a sub-tool of the pen tool.
in osX: They may have changed it to "Command" key.
SO simple - THANK YOU
Still helpful in 2017. Using Ctrl key on PC.