Kaishō (open hand) is more than just an open hand. In Karate, several techniques are used with the open hand. The position and tension of the hand muscles depend on the technique you want to perform.
To perform it properly, in most techniques the palm should be tensed forward, the fingers should be clenched and slightly flexed, and the thumb should be held strongly against the inner edge of the hand.
In most Kaishō techniques, the back of the hand is in line with the forearm, just as in most Tsuki techniques.