I updated the title of your question from “slow down” to “reversal”. Seems to me from your screenshot that is more in line with your request.
You will find the code I provided for our MTF MACD scan will do exactly this. Follow the steps in this article but only apply it to a signal time frame. Code is included: https://www.hahn-tech.com/thinkorswim-mtf-macd-scan/
Update: 10/29/18
After receiving additional details I have the following code to offer.
input fastLength = 12;
input slowLength = 26;
input MACDLength = 9;
input averageType = AverageType.EXPONENTIAL;
def Diff = MACD(fastLength, slowLength, MACDLength, averageType).Diff;
plot scan = Diff > Diff[1] and Diff < 0;