Two-isogeny descent
= Two-isogeny descent
Here descent is an arithmetic calculation with square classes and an <isogeny of elliptic curves>, rather than the gluing of objects along a covering.
For $E:y^2=x^3+ax^2+bx$ and its two-isogenous curve $E':y^2=x^3-2ax^2+(a^2-4b)x$, square-class maps on $E(\mathbb Q)$ and $E'(\mathbb Q)$ determine the Mordell-Weil rank.