The "Home-Run Derby Slump" refers to the supposed curse that players who participate in the Home-Run Derby perform more poorly in the second half of the season. For example, Todd Frazier's OPS in 2015 before the event was .782, compared to .749 after the event.
This year's derby winner (Aaron Judge) already has a high OPS of 1.139, giving him more potential to fall harder.
Today, we asked people if Aaron Judge's OPS over the next four weeks will be above or below .950. (From July 14-August 11)