Central Simple Algebras with Involution: A Geometric Approach
  Let $k$ be an algebraically closed base field of characteristic zero. The category equivalence between central simple algebras and irreducible, generically free $PGL_n$-varieties is extended to the context of central simple algebras with involution. The associated variety of a central simple algebra with involution comes with an action of the semidirect product $P_{n,\tau}:=PGL_n\rtimes<\tau>$, where $\tau$ is the automorphism of $\PGLn$ given by $\tau(h)=(h^{-1})^transpose$. Basic properties of an involution are described in terms of the action of $P_{n,\tau}$ on the associated variety, and in particular in terms of the stabilizer in general position for this action. 
Nikolaus Vonessen