Billionaire George Soros' family office hedge fund, Soros Fund Management, filed its 13F quarterly report with the Securities and Exchange Commission yesterday.

As Marketwatch reporter Barbara Kollmeyer points out, one interesting highlight from Soros' filing is that he bought a bunch of puts on the SPDR S&P 500 ETF in Q2.

It's his biggest holding in the filing.

During the second quarter ended June 30, Soros held 26,157 shares of SPDR S&P 500 and call options on 143,600 shares and put options on 7,802,400 shares in the ETF.

In the first quarter ended March 31, Soros held 17,065 shares and puts on 2,618,700 shares of SPDR S&P 500 ETF.

What's so significant about this move is that puts are used for a downside bet.

It appears that Soros has placed a large bet through S&P 500 puts, basically giving him the right, but not the obligation, to sell them in the future.

So if the S&P 500, or the ETF which tracks the S&P 500 goes down, Soros will profit handsomely.

Then again, Soros also bought 66,800 shares of Apple (a major component in the S&P) and he owns a bunch of other stocks. So buying S&P 500 puts can also act as a hedge.

Also, keep in mind that these 13F filings come 45 days after the end of the quarter. Fund managers only have to disclose their long equity holdings in these 13Fs. We don't know what he's shorting.