Deflagration to detonation transition requires formation of a shock at least as strong as Chapman-Jouguet. Chemistry ahead of a flame, in the finite zone of reactive mixture behind the precursor shock, or, more generally, behind a source of an unburned mixture, always results in a hot spot appearing ahead of the flame. Diffusion-driven flames propagate at negligible speeds compared with the speed of sound and can thus be approximated as contact surfaces. Downstream, the hot spot triggers a weak detonation that slows down to the Chapman-Jouguet speed and quickly becomes a strong wave. Eventually, this wave reaches the contact surface; chemistry stops and a reflection moves upstream. If the source delivers fresh mixture at a velocity below a specific threshold, then the hot spot does not initially trigger a combustion wave on its upstream side, but only an inert shock. This shock, however, warms up the mixture, resulting in a much shorter ignition time. If ignition behind this second shock occurs before the reflection of the downstream wave on the contact surface reaches the location of this second ignition point, a second hot spot forms, on these much shorter time and length scales. This second hot spot triggers a second identical chain of events, which triggers a third one, and so forth, resulting in an embedded sequence of ignition and shock formation. Each of these appears slightly further downstream, on increasingly shorter scales, triggering the next events in the sequence. Eventually, a sequence of shock mergers results in a strong Chapman-Jouguet wave, completing transition to detonation. The condition for reflection to take longer than ignition yields a transition criterion. The analysis assumes high activation energy and a ratio of specific heat close to unity, and the transition criterion appears as a specific distinguished limit.