  The extremely rapid burst of TeV photons from Mkn 421 (15 May 1996) can be
reconciled with the standard properties of a relativistic gamma-ray emitting
jet (bulk Lorentz factor $\approx$ 10; size $\10^{17}$ cm) if one assumes that
the electrons are accelerated in conical shocks with both opening and viewing
angles $\approx 1/\Gamma$. If the injection time and the cooling time are much
less than the photon crossing time, an emission ring moves along the jet and
leads to the appearance of a very rapid flare, in satisfactory agreement with
the observations.
