I was looking into the classification of real simple lie algebras from Araki (1962). In page $4$, proposition $1.1$, a criteria is given as to when $\mathfrak{h}^-$ is maximal abelian in $\mathfrak p$ . I could not figure out things from the reference he has given. I started playing around with lie brackets and applying $\sigma$ and $\tau$ in different ways but reached nowhere. Can someone please help me in that direction?
Any help would be appreciated. Thanks in advance.