While every good hook starts at least somewhat in media res (in the middle of the action) whether it is as the main character ends the mysterious town or after the strange things begin, the Sudden Grasp hooks make that most obvious. It begins with a bang! Something happens and the protagonists (the player characters) are put on the defensive. You don’t have time to think about it. You just have to react and hope you make it out alive.
Players often really enjoy this hook simply because it’s fairly uncommon. Even videogames normally prepare you with some form of cutscene and roleplaying games often stretch it out all the more with tavern scenes, meetings on dark rooftops, or some other gentler example.
The Sudden Grasp can begin with sudden violence and an attempt to flee or chase someone down like in the Getaway, but it can just as well begin the way Fahrenheit (Indigo Child) does with the protagonist immediately having to cover up a murder scene. What you do here is you give the players some form of reactive objective they have to complete. Bonus points if there’s a time limit on it (sinking ship, ticking time bomb, SWAT arrival).
The trick with these adventures is to take it off the tracks. You really can’t railroad players as planning and cautious thought will fly out the window when the PCs aren’t allowed to feel comfortable, get to know each other and their surroundings. Who knows what they will do? You certainly won’t until they do it.
Also be prepared to still spend a minute or two describing the scene. Who’s there? Where is it? Why are the PCs there?
Your best bet is to keep the motivations simple. Beginning with the Sudden Grasp when they’re in the middle of a drug deal gone sour is very difficult. You’d be better off taking a little time to let them arrive, talk amongst themselves, and chat to the dealers. So keep it very simple with an obvious goal – such as the PCs are all walking through the park on their way somewhere when a bomb goes off or a werewolf pack descends on them all.
If it’s the start of a campaign or new adventure, then the Players don’t even really know their PCs too well so you’re better off not going with hooks involving their children going missing unless they’re very seasoned at jumping into their character’s skins on the fly. If they’re playing existing characters, then it’s a little easier and they can jump into the middle of more complex tales with more complex motivations so long as the setting details themselves are kept rather simple.
But again, take it off the rails. No one can truly predict themselves or other people in a crisis situation. Players are no different.