It is most likely a common knowledge already, but what the hell, I'm a slowpoke.
Tail Flip is -7 on block and considered "safe" against majority of the cast since Alien can poke after and iterrupt any non armor attempts to counter attack. Liu Kang has 6 frame normal which is f1. It takes 1 frame to release the block and then you have 6 frames to punich Alien's Tail Flip. The timing is difficult, but payback is nice.
You can punish it the same way with Raiden since he also has 6 frames normal which is f1 also.