  Context: Active M dwarfs frequently exhibit large flares, which can pose an
existential threat to the habitability of any planet in orbit in addition to
making said planets more difficult to detect. M dwarfs do not lose angular
momentum as easily as earlier-type stars, which maintain the high levels of
stellar activity for far longer. Studying young, fast-rotating M dwarfs is key
to understanding their near stellar environment and the evolution of activity
Aims: We study stellar activity on the fast-rotating M dwarf GJ 3270. Methods:
We analyzed dedicated high cadence, simultaneous, photometric and
high-resolution spectroscopic observations obtained with CARMENES of GJ 3270
over 7.7 h, covering a total of eight flares of which two are strong enough to
facilitate a detailed analysis. We consult the TESS data, obtained in the month
prior to our own observations, to study rotational modulation and to compare
the TESS flares to those observed in our campaign. Results: The TESS data
exhibit rotational modulation with a period of 0.37 d. The strongest flare
covered by our observing campaign released a total energy of about 3.6e32 erg,
putting it close to the superflare regime. This flare is visible in the B,V, r,
i, and z photometric bands, which allows us to determine a peak temperature of
about 10,000 K. The flare also leaves clear marks in the spectral time series.
In particular, we observe an evolving, mainly blue asymmetry in chromospheric
lines, which we attribute to a post-flare, corotating feature. To our knowledge
this is the first time such a feature has been seen on a star other than our
Sun. Conclusions: Our photometric and spectroscopic time series covers the
eruption of a strong flare followed up by a corotating feature analogous to a
post-flare arcadal loop on the Sun with a possible failed ejection of material.
