### Comment:
But ETFs like SPY have scheduled rebalancing and don't rebalance in real time. 
  
 I don't think the ETF is buying/selling shares, but it makes sense if the market maker is buying/selling shares of the underlying with an equal weight of the etf when buy/sell orders come in. That's probably the answer.

### REPLY:
The company that runs the ETF definitely have strategies to prevent their ETF from trading at a steep discount. They face steep losses if they allow their ETF to trade at a steep discount. 
  
  
 https://www.schwabfunds.com/public/file/P-11570533

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