Actually, you can not have a completely tight fist while having a relaxed wrist. It is anatomically impossible. It has to do with the fact that the finger flexors and extensors share a common origin with the wrist flexors and extensors. If the fingers are tight, the tendons cross the wrist and it is tight. That is why a person can not grip well when they have "tennis elbow".
The concept of the ureken and why you can have a tight fist at impact with bending wrist is due to tendon insufficiency. The tendons of the fingers are not long enough to allow both wrist flexion and finger flexion (or extension of both). If a person flexes the wrist, they can not make a fist (that is why wrist locks are good for disarming knives). However, when the wrst bends back into an ureken, the fingers "want" to bend and make a fist. It does not take a lot of extra effort to tighten them into a strong fist.