The maximum amplitude comes when contraction frequency becomes more frequent, causing contractions to have their amplitudes add together.
Work Step by Step
To see this, look at Fig. 46.18 and the text on p. 975. The summated contraction in Fig. 46.18 has a peak and then another peak, with multiple ones like this in the tetantic contraction in that figure. The text makes it clear that this comes from contractions adding on top of one another like waves in the ocean adding together to make one large rogue wave.