  Fine-scale braiding of coronal magnetic loops by continuous footpoint motions
may power coronal heating via nanoflares, which are spontaneous fine-scale
bursts of internal reconnection. An initial nanoflare may trigger an avalanche
of reconnection of the braids, making a microflare or larger subflare. In
contrast to this internal triggering of subflares, we observe external
triggering of subflares in a braided coronal magnetic field observed by the
{\it High-resolution Coronal Imager (Hi-C)}. We track the development of these
subflares using 12 s cadence images acquired by {\it SDO}/AIA in 1600, 193, 94
\AA, and registered magnetograms of {\it SDO}/HMI, over four hours centered on
the {\it Hi-C} observing time. These data show numerous recurring small-scale
brightenings in transition-region emission happening on polarity inversion
lines where flux cancellation is occurring. We present in detail an example of
an apparent burst of reconnection of two loops in the transition region under
the braided coronal field, appropriate for releasing a short reconnected loop
downward and a longer reconnected loop upward. The short loop presumably
submerges into the photosphere, participating in observed flux cancellation. A
subflare in the overlying braided magnetic field is apparently triggered by the
disturbance of the braided field by the reconnection-released upward loop. At
least 10 subflares observed in this braided structure appear to be triggered
this way. How common this external trigger mechanism for coronal subflares is
in other active regions, and how important it is for coronal heating in
general, remain to be seen.
