It's been a long time since the last 400 SMA test for the S&P. It tried to test the 400 the last time that price dipped well into the 1 standard deviation band (for the 400 SMA), represented by the purple band closest to the dark purple line (400 SMA), but failed to touch it (came damn close, though).
BTW, each colored band represents 1 more standard deviation out from the mean. There are 5 bands in total, representing from 1 to 5 standard deviations away from the mean - 400 period simple moving average.
Historically, price trades between the first and second standard deviation bands for most of an uptrending period, as has happened here. However, with the rounded top and failure to put in a high as close to the second standard deviation level as were previous highs, this is showing all the signs of a market that has lost upward momentum and in need of a major support level.
The short signal happened when price closed below the 1 SD level. The first target is the major dynamic support of the 400 SMA. If that busts, we should at least get another test of it from underneath. If it busts and holds as resistance, then we're in trouble, IMO.