  A nonperturbative theory of multiphonon anharmonic transitions between energy
levels of a local mode is presented. It is shown that the rate of transitions
rearranges near the critical level number $n_{cr}$: at smaller $n$ the process
slows down, while at larger $n$ it accelerates in time, causing a jump-like
loss of energy followed by the generation of phonon bursts. Depending on
parameters, phonons are emitted in pairs, triplets etc.
