Timestamps:
00:01 PJWS passive descriptions
00:06 Zhongli attack with 0 stacks
00:31 Zhongli attack with max stacks
TL;DR
The passive works on your attack hitting the enemies, not on the enemies hitting on you. You also can use your summoned unit such as zhongli pillar or any skills to hit the enemies and the stack will accumulate. Though you need your character using the weapon to be on field to get the stacks.
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