### Comment:
No it’s almost immaterial how much SPY moves the S&amp;P because the volume on SPY is a tiny fraction of the volume on the S&amp;P constituents.  In theory it does move it a little, but in practice it’s almost entirely the other way around (participants adjust the price of spy to follow S&amp;P)

### REPLY:
&gt; volume on SPY is a tiny fraction of the volume on the S&amp;P constituents  
  
 That's why it seemed off to me.  
  
 But is he right in saying that the SPY and other index based ETFs buy and sell their underlying constantly throughout the day? 
  
 &amp;#x200B;

### END.