  We aim at advancing our understanding of magnetic activity and the underlying
dynamo mechanism at the end of the main-sequence. To this end, we collected
simultaneous X-ray and radio observations for a sample of M7-L0 dwarfs using
XMM-Newton jointly with the JVLA and the ATCA. We also included data from the
all-sky surveys of eROSITA on board the Russian Spektrum-Roentgen-Gamma mission
(SRG) and rotation periods from TESS together with an archival compilation of
X-ray and radio data for UCDs from the literature. We limit the sample to
objects with rotation period <1d, focusing on the study of a transition in
magnetic behavior suggested by a drastic change in the radio detection rate at
vsini \approx 38 km/s. We compiled the most up-to-date radio/X-ray luminosity
relation for 26 UCDs with rotation periods lower than 1d, finding that rapid
rotators lie the furthest away from the G\"udel-Benz relation previously
studied for earlier-type stars. Radio bursts are mainly experienced by very
fast UCDs, while X-ray flares are seen along the whole range of rotation. We
examined the L_{\rm x}/L_{\rm bol} vs P_{\rm rot} relation, finding no evident
relation between the X-ray emission and rotation, reinforcing previous
speculations on a bimodal dynamo across late-type dwarfs. One radio-detected
object has a rotation period consistent with the range of auroral bursting
sources; while it displays moderately circularly polarized emission. A radio
flare from this object is interpreted as gyrosynchrotron emission, and it
displays X-ray and optical flares. We also found a slowly rotating apparent
auroral emitter, that is also one of the X-ray brightest radio-detected UCDs.
We speculate that this UCD is experiencing a transition of its magnetic
behavior since it produces signatures expected from higher mass M dwarfs along
with emerging evidence of auroral emission.
