See hou.FlipbookSettings.motionBlurFrameRange().
Values
hou.flipbookMotionBlurBias.Centered
Subframe range is centered around the currently rendering frame.
hou.flipbookMotionBlurBias.Forward
Subframe range begins at the currently rendering frame.
hou.flipbookMotionBlurBias.Previous
Subframe range ends at the currently rendering frame.