Flurry kill? And what is a head explosion?

[Spifff]

When i first got the wraith stun (punch a dude and he gets stunned with a blue aura) and flurry i would stun and then mash the attack button. i'd do around 8 of those rapid attacks and then grab the orc by the throat and explode his head.

Now when i do a flurry (50% done game) all the way through i just end up smacking the orc with a backhand at the end of flurry which causes him to fall to the ground. Sometimes not even killing him. I've tried timing my flurries (for the extra hit streaks with timed button presses) or just mashing it as fast as a can. NEVER does it do the neck grab head explosion thing. Was i imagining it in the past or am i doing something wrong?

That question then leads me to more questions about "Head Explosions". I have a runes that mentions regenerating some resource on head explosion kills. But how does one force a head explosion? I first remembered my flurry thing but then after experimenting i can't seem to make it ever happen. And i think sometimes i see heads explode at random but i'm not 100% sure anymore. Is it just random?

Most of my fights involve building hitstreak by 5, double execute, build hitstreak by 5, double execute, repeat.

Any thoughts appreciated.

[Coookie]

There's an upgrade that causes the exploding.