The phrase "second derivative" refers to the rate of change of a rate of change. It measures how quickly the rate of change of something is changing. Full definition
Since prepayments typically rise as interest rates fall and vice versa, the basic (pass - through) MBS typically has negative bond convexity (second derivative of price over yield), meaning that the price has more downside than upside as interest rates vary. (en.wikipedia.org)
In the words of MX - 5 ambassador Nobuhiro Yamamoto, considered by many the godfather of the MX - 5, the RF is an important second derivative. (motoring.com.au)
Look at the dT / dt and second derivative curves (diagno.png). (judithcurry.com)