Hey Stormey.
I'm not certain but I'm thinking that it means that <Tu,Tv>/|Tu|*|Tv| = <u,v>/|u||v| where <.,.> is an inner product and |.| is a norm = SQRT(<.,.>)
I read somewhere that normal transformations (hermitian, unitary, orthogonal, etc...) preserves the angles between vectors.
but transformation is a function that operates on one vector, not two ( ), so what angle exactly are we talking about here?