Buy S&P 500 @ 1897, Sell S&P 500 @ 1900- win

Buy S&P 500 @ 1897 if it stays above & goes higher, for 30 minutes expiration
Sell S&P 500 @  1902 or 1900 if it stays below & drops further, for 30 minutes expiration

Result: win
Buy S&P 500 @ 1897 after it stayed above & went higher, avoid buying when it dropped below 1897
Sell S&P 500 @ 1899 at higher risk since it stayed below 1899 & couldn't even reach 1900 anymore