How does the Ethereal's damage reflection work?

From my experience it seems to be a random chance to block all shots. But I am not able to tell you the exact % - I estimate it is around 20% from my own experience.

I have had the following shots blocked:

• 100% chance to hit flanked shotgun at point blank range
• Sniper 'headshot' from a long range and a normal sniper shot
• Normal shots

I have also seen the reflection

• On the first shot fired at him
• On full health
• On 1 health
• On half health
• On the 2nd,3rd and 4th shot fired at him (all in the same turn - i.e. he can do it more than once a turn)

What I have NEVER seen "reflected" is rocket and grenade damage. So if the Ethereal is on low HP - it might be better to kill him this way to guarantee yourself the kill. Likewise (if riskier), it can't block the Arc Thrower, and the upgraded one has a high chance of a stun on 6 health and below.