In Oxford Primer:

Pericyclic Reactions by Ian Fleming it is written that the deuterium substitution of the hydrogen atoms in the diene and dienophile that participate in a possibly Diels-Alder reaction, gives rise to inverse secondary isotope effect (as the carbon atoms change from trigonal to tetrahedral geometry):

-If both bonds are forming at the same time, the isotope effect when both ends are deuterated is geometrically related to the isotope effect at each end.

-If the bonds are being formed one at a time, the isotope effects are arithmetically related.

I don't understand the principle behind this. Why does the isotope effect occur and how does it tell us whether the mechanism is radical or pericyclic? I need an answer much more concise than what is presented in the book.