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During the last decade, M87's jet has been the site of an extraordinary variability event, with one knot (HST-1) increasing by over a factor 100 in brightness. Variability has also been seen on timescales of months in the nuclear flux. Here we discuss the optical–UV polarization and spectral variability of these components, which show vastly different behavior. HST-1 shows a highly significant correlation between flux and polarization, with P increasing from∼ 20% at minimum to> 40% at maximum, while the orientation of its electric vector stayed constant. HST-1's optical–UV spectrum is very hard (α UV–O∼ 0.5, F ν∝ ν− α), and displays" hard lags" during epochs 2004.9–2005.5, including the peak of the flare, with soft lags at later epochs. We interpret the behavior of HST-1 as enhanced particle acceleration in a shock, with cooling from both particle aging and the relaxation of the compression. We set 2σ upper limits of …