Stand in the flowers corner next to boiler exit. Aim at the tree above balcony, at the bottom of the small gap in the leaves. Regular throw.
The flash will explode right at the tip of the balcony, which will pop flash any CTs in pit but will not blind your teammates moving up arch.
Pro example: Team Spirit against Vitality https://youtu.be/bptfAhxIWsk?t=234
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