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abxPlayKeys is a way of previewing your animation in stepped mode without ever altering your keyframe tangents. Select the animated object(s) so their keyframes are visible in the timeline and then run abxPlayKeys to see the animation play in stepped mode. You must call the script each time you want to see your animation play in stepped.
It's like pressing the '>' (next Keyframe) key through your animation only it does so in real-time.