The big themes of sweeping and hooking, in theory, are, I believe:
- Sweeping endeavors to create a sticky, scraping force and moves in a wheeling fashion. An opponent hit by a sweeping strike, particularly one that glances off or that is used to open the opponent, should be dragged off balance a little by the strike. This is accomplished by an idea of wheeling and scraping. Sweeping strikes also hit directly, cutting into the opponent like a sword.
- Hooking endeavors to move the opponent as if they were being snagged by a big hook. The name is dual in meaning: the arm is shaped like a hook and the arm is used like a hook. To use a hook well, one would have to stretch out and then come back, push and then pull, if you will. The idea is that the arm should be carrying the opponent somewhere as if he's been snagged by a hook, so there's a real idea of moving the opponent around with a hooking strike.
In the second example, there's an application that's relatively accessible using any of rising sweeping, rising cutting, or opening hooking (the first and last on this list being quite similar in execution with that same slight difference) where the opponent's arms end up crossed while you stand behind him and put pressure on his arm and throw him down. It's in the Forcing Hand dvd, for instance, but the exact move isn't important to this discussion. Doing that move with sweeping until it's relatively comfortable, then with hooking until the same, and then going back and forth really underscored the difference. Hooking has a very strong "I'm carrying this guy around" feeling to it where as sweeping does not. Sweeping has more of an idea of knocking the guy out of the way but in a manner where there's still connection.
I don't think this is very clear, but I'm going to let it stand. It's quite difficult, I'm sure you can appreciate, to discuss movements and kinesthetic sensations in text, and as far as applications go, they never come out well when written down (at least I don't seem to think they do). The point is that the two palms feel different even though they have techniques that are done very similarly, and that those similarites boost development in the gross skills while those differences hone the mind to pay attention to and use subtle differences to achieve different results in the same or different situations. As far as I know, the only way to "get" it is to realize there's a difference, train how you believe that difference might manifest, try it out, revise, refine, and then train it and try it again and again. Baguazhang is not redundant (that would be inefficient and unneccessary), and so if they feel the same to you, then you're doing something wrong, i.e. it's incorrect to say or think or feel that "opening hooking is just like rising sweeping with a closed fist and slightly more bent arm." While on the outside, gross level that's largely true, there's different intent there that can only come about by looking for, focusing on, and then training the subtle difference in strategy and technique between the two strikes. Still, the similarity is strong enough to achieve the following two goals, in my experience: 1) getting beginners started with the hooking strike, and 2) to have a strong overlap in the "finding the force" effort in both strikes.