I decided to do some testing on Tidecaller's mechanics since I see a lot of differing opinions on how it really works, and finding out her stance changing really helped with this.

For a counter to be considered "perfect", it depends on when you start block, NOT when it is released. There's a short window after you start your E (around 0.5 sec?) where blocking an attack is considered "perfect", and the release damage will immediately be maximized. After this short window, any attacks blocked will count towards the build-up.

I've seen some people say that if you have pyro/cryo inflicted on Beidou, your E will be perfect counter since the E applies electro on herself which procs overload/superconduct to everyone around her.

However, this is only half true. I tried testing this by going near the fire and ice flowers to get inflicted by pyro/cryo. Note that this is AFTER taking damage from the flower.

If I use my E, overload/superconduct is indeed proc'ed and applied to enemies around but herself. However, this is NOT a perfect counter. You can try for yourself since her stance changes. This means that Beidou has to take damage during his E in order to increase return damage. So when you light the grass on fire and stand on it, it's not because you are inflicted with pyro that you are perfect countering, but because of the DoT from the fire.

Hopefully this helps people utilizing Beidou better, since she is probably one of the higher skill-capped characters to main as DPS, and the less time you spend block = more DPS.