So i was playing on my lvl 53 priest in Un'Goro. While killing some mobs i used a rotation of Vampiric Touch, Mind Blast, Shadow Word - Pain. That gives me 3 stacks of Shadow Weaving, that increases shadow damage done by 6%. After this i used ONE mind flay, and it immediately gave me full 5 stacks of shadow weaving! So i went ahead and tried only Mind Flay and it always gave me 2 stacks per one cast.
I have pics aswell
Here it is in 3 stacks ->
Here in 5 stacks after one mind flay ->
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