Kake-dachi, the hook stance is a transitional stance, used to setup 180° turns or spin kicks. Other karate style refer to this a kosa-dachi, or crossing stance.
In a proper hook stance, the front foot remains flat on the ground and bears 90% of your bodyweight, with the knee slightly bent. The rear foot “hooks” the supporting ankle (thus, the name), and rests 45° outside and behind it. The rear heel is raised, so only the toes and the ball of the foot contact the ground. The rear foot only carries 10% of your bodyweight.
Phrased differently, stand in a cat stance and turn 180° towards your supporting leg.
[photo of hook stance from the front and side.]
Do not remain in a hook stance for any prolonged time; your balance can be compromised more easily from this position. However, any time you need to turn around, there will be a brief moment spent in a hook stance.