Daiwa Ryu jujutsu, like many other jujutsu and aikido styles, has a number of kata techniques applied from wrist grabs. At first this seems incredibly artificial - just who in their right mind is going to grab your wrist as an attack?
As I've continued training, I have justified this to myself in a number of different ways. At first I assumed it was just a teaching tool, learn how to apply the technique from a wrist grab, then move to alternative attacks. Then I thought these techniques might be an anachronism, left over from when you might want to prevent an opponent drawing a sword, so hand control was vital. I have now come round to the opinion that these wrist grabs are actually valid attacks in their own right.
So what's changed my mind? It's the way that you make that wrist grab that transforms it from an ineffective teaching tool to a genuine attack. First consider the grip itself. If you look at the palm side of your hand whilst it opens and closes, you'll notice that the little finger through to middle finger close naturally against the base of the thumb. The index finger closes against the thumb. It is this opposable thumb that gives us great manual dexterity, but it's not what you're looking for when you try to grab onto someone. The power of the grip comes from the last three fingers, tightened up from the little finger first. Meanwhile the thumb and index finger remain loose and sensitive so that you can feel what's happening.
Next consider where to grab. In a typical cooperative training environment it is common to see the grip applied to the lower forearm of tori. From here it is incredibly difficult to affect your opponent's posture - all you can do is wait passively for them to take the role of tori and apply their technique. Instead try gripping actually at the wrist - the narrowest part, between the hand and forearm. Unlike the mostly featureless cylinder of the forearm, there are lumps and corners in the wrist that give you something to hold on to. You are also splinting the joint, reducing your opponent's freedom to move.
So you've now got a strong grip established on a grippable part of your opponent, a good start but still not a valid attack. You need to use that point of contact to affect your opponent's balance. Like any other techniques, as you explore pushing and pulling in different directions, you will find some that work better than others. Too hard to describe in words here, the best advice is just to practice.
Next time you grab someone's wrist, don't just passively wait for them to apply their technique, see what technique there is in that grab for you.