They mark either the highs below highs or the low above lows on the NYMO. If you study the chart, matching up the highs and lows with the price of the SPY you may see the NYMO gives advanced notice of the NYSI turn up or down. The NYMO can be traded by aggressive swing traders with hard stops. The NYSI is more often the signal for a sustained broad market rally or decline.

Thanks for this question. It has prompted me to incorporate my response into the body of the post, buth here and in my journal at Substrack.