  A reversal of the particle current in rocking Brownian motors was predicted
more than 20 years ago; however, an experimental verification and a deeper
insight into the underlying mechanisms remained elusive. Here, we investigate
the high frequency behaviour of a rocking Brownian motor for charged
nanoparticles based on electrostatic interactions in a 3D shaped nanofluidic
slit and electro-osmotic forcing of the particles. A sub ms temporal and
$\approx\,10\,$nm spatial resolution of the 60 nm gold spheres allows us to
measure the time-resolved and frequency dependent evolution of the particle
probability density in-situ. At 250 Hz the particle current changes sign, in
agreement with a theoretical model based on the time-dependent Fokker-Planck
equation. From this fit-parameter free description and its excellent agreement
with the observed behaviour, we trace the origin of the current reversal to the
asymmetric and increasingly static probability density at high frequencies.
