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First tutorial here: http://youtu.be/HFDVybD0QAs
F5 : Extend hold, make the entire animation or just the selected layer longer
F6 : Make a Keyframe. If there is a hold, it'll make a new frame keeping the current drawing intact.
F7: Make a blank keyframe (won't delete anything from just a keyframe, must be a hold.)
SHIFT+F5: Remove frame, making just the layer or entire animation shorter
SHIFT+F6: Clears the keyframe