What I observed is that on 2 occasions, (points which are marked with arrows as A and B), the SPX went above the Upper Limit and then immediately fell back to within the channel.
At Point A, it returned back to the Median and then followed a gradual upward path towards Point B, whereupon, the SPX Monthly then fell sharply back to the Median and continued to fall to below the edge of the Lower Limit.
Event A took place in the month of Jan 2018 and Event B subsequently worked itself out in the month of September 2018.
The SPX Monthly is now again back up and beyond the Upper Limit of the Regression Zone and, if; history repeats itself, then the odds are high that it will exercise Reversion to The Mean again on this 3rd occurrence.
I link into a previous post , (about Chart Patterns that tend not to fill) and am of the opinion that 3102 on the SPX will be the Maximum Floor towards which the SPX will revert back to.
This is still very much a powerful Bull Market and the Top Is Not In, and may not even be in till perhaps closer to 4000+ (as the Market nears the results of the US Presidential Elections of 2020 in November).