  We observed rapid variability ($\lesssim 60$ s) at the footpoints of
transient hot ($\sim 8-10$ MK) coronal loops in active region cores, with the
Interface Region Imaging Spectrograph (IRIS). The high spatial ($\sim 0.33$
arcsec) and temporal ($\lesssim 5$-10 s) resolution is often crucial for the
detection of this variability. We show how, in combination with 1D RADYN loop
modeling, these IRIS spectral observations of the transition region (TR) and
chromosphere provide powerful diagnostics of the properties of coronal heating
and energy transport (thermal conduction and/or non-thermal electrons (NTE)).
Our simulations of nanoflare heated loops indicate that emission in the Mg II
triplet can be used as a sensitive diagnostic for non-thermal particles. In our
events we observe a large variety of IRIS spectral properties (intensity,
Doppler shifts, broadening, chromospheric/TR line ratios, Mg II triplet
emission) even for different footpoints of the same coronal events. In several
events, we find spectroscopic evidence for NTE (e.g., TR blue-shifts and Mg II
triplet emission) suggesting that particle acceleration can occur even for very
small magnetic reconnection events which are generally below the detection
threshold of hard X-ray instruments that provide direct detection of emission
of non-thermal particles.
