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Well obviously rebalancing is not the only time etfs transact. They trade (both ways) baskets of index constituents vs own shares with select counterparties every day as per their AP requests. APs are the ones tracking NAV deviation. See etf creation/redemption process.
Oh sry.
But imagine a theoretical - all active players start selling, S&P plummets, but all etf hodlers hodl. No in/outflows. You saying no selling happens? So what does happen? 
Buying in the SP500 happens. The APs in this case would sell etf shares (which trade at a premium to NAV) and buy index constituents. However as we've seen in march, it's the effs that get dumped first, like HYG, which traded at a big discount, if not mistaken.
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