Inflation is the first derivative. The rate of increase of inflation is the 2nd derivative. The idea that the rate of increase of deflation is itself decreasing would then be the 3rd derivative (think "the rate of the rate of increase of inflation is negative")

money = money(t)

inflation = d money / dt

rate of increase of inflation = d^2 money / dt^2

rate of change in rate of increase of inflation = d^3 money / dt^3

rate of increase of inflation was decreasing = sign(d^3 money / dt^3)

Huh?

I parse it as,

money = money(t)

inflation = d money / dt

rate of change (increase or decrease) on inflation = d^2 money / dt^2

^^^^ and this is the one w/ a low value

It doesn't have a low value, but it itself is decreasing, thereby referring to money's third derivative.

Aaaaaah, OK, I see it now.

I believe they're counting inflation itself as a derivative