  We study spectroscopic observations of chromospheric evaporation mass flows
in comparison to the energy input by electron beams derived from hard X-ray
data for the white-light M2.5 flare of 2006 July 6. The event was captured in
high cadence spectroscopic observing mode by SOHO/CDS combined with
high-cadence imaging at various wavelengths in the visible, EUV and X-ray
domain during the joint observing campaign JOP171. During the flare peak, we
observe downflows in the He\,{\sc i} and O\,{\sc v} lines formed in the
chromosphere and transition region, respectively, and simultaneous upflows in
the hot coronal Si~{\sc xii} line. The energy deposition rate by electron beams
derived from RHESSI hard X-ray observations is suggestive of explosive
chromospheric evaporation, consistent with the observed plasma motions.
However, for a later distinct X-ray burst, where the site of the strongest
energy deposition is exactly located on the CDS slit, the situation is
intriguing. The O\,{\sc v} transition region line spectra show the evolution of
double components, indicative of the superposition of a stationary plasma
volume and upflowing plasma elements with high velocities (up to
280~km~s$^{-1}$) in single CDS pixels on the flare ribbon. However, the energy
input by electrons during this period is too small to drive explosive
chromospheric evaporation. These unexpected findings indicate that the flaring
transition region is much more dynamic, complex, and fine-structured than is
captured in single-loop hydrodynamic simulations.
