Take an index and strip out the weights: give every member the same vote and average their returns. That is the equal-weight version, and the gap between it and the real index is one of the most information-dense numbers in a session.
When the cap-weighted S&P 500 rises 0.72% and its equal-weight version rises 0.70%, the day's gain was democratic; the typical stock earned roughly the headline. When the index rises 0.72% and equal-weight is flat, the entire gain came from size: the biggest names went up and everyone else, on average, did nothing. Persistent gaps in one direction describe a market where leadership keeps narrowing or broadening, which matters more than any single day.
The equal-weight line answers the question a headline never does: was that the market, or was that five stocks? WMTMT draws it on every chart as the dashed overlay and states the gap in the daily notes.