George, a "crushing grip" and making your partner's hand turn purple doesn't imply tensing up one's arm.
Ok Demetrio, I have no idea how you would accomplish that without tensing up your arm. Maybe you have a secret I do not.
However, I will say that everyone I have ever trained with, which is of course limited to my own personal experience but includes folks from all sorts of Aikido stylistic backgrounds, who crushed my wrists, was tense and was limiting his own freedom of movement in trying to limit mine. If we ever get together you can show me how you'd do it without tension.
My general rule of thumb is that the place at which you feel the power is the place at which they are putting their energy and their attention. When I grab someone I want them to feel like I grabbed their center and I do not want them feeling much at the wrist. It is a waste of energy and has no function.