What astronomers call "He flash" is the step in evolution of small (1-2 Sun masses) star when degenerate core accumulates enough He to quickly burn it into C in a matter of a few seconds. Because core is degenerate (electron pressure does not rize with the rize of temperature), core does not swell at the beginning of a flash to slow it down by dropped density, and almost all He core burns in a matter of few tens of seconds. Imagine that the amount of energy released in normal circumstances in few billion years is now released in such short time. Core brightness is about 10^14 times Sun brightness - so it is like a supernova birth, but INSIDE thick H outer layer of the star. This layer completely blocks the explosion, absorbing all energy (but neutrinoes) and all visible traces of enormous energy release. So, He flash goes completely unnoticeable for outside observer. As far as I know, there are no experimental evidence of a He flash so far (due to thick H shell of stars absorbing all energy of deep inside processes), although most star evolution models predict He flash.

Interesting, that M27 is not a real planetary - creating nebulae - it is just a bunch of ionized H gas blown by and heated by a brightening central star which is approaching its death as it beging burning heavier than H elements.