Well, I talked just a little about bagua (pa kua) yesterday, so I thought I'd give a nod to xingyi (hsing-i) today.
Xingyi is one of the three traditional Chinese Internal martial arts: the other two are bagua (pa kua) and taiji (tai-chi). They are three different interpretations of the principles of "internal energy" in fighting. All three arts are "internal": they all work on the principle of relaxation and breathing. But they take different approaches to interpret those principles.
In general, xingyi is linear: the practitioner moves in roughly straight lines, striking and kicking to attack the opponent's structure. Bagua is generally circular, the practioner moves in a circular pattern, typically around the opponent. Taiji is somewhat of a spiral, where the practioner moves more or less from one point.
These, of course, are generalizations. There are certainly linear bagua sets, just as there are spinning, circling xingyi techniques.
As far as aggression, xingyi is about the attack; taiji is about reaction to an attack; bagua is somewhere in the middle. Bagua has a strong focus on evasion, taiji is about "yielding", xingyi is about crushing.
Traditional wisdom says aikido is based on bagua, karate on xingyi.
Xingyi is based on five motions: splitting, drilling, crushing, pounding, and crossing. These "Five Elements" correspond to the alchemist elements of: metal (splitting), water (drilling), wood (crushing), fire (pounding), and earth (crossing). Here's a video of a xingyi practitioner practicing the Five Elements:
That style looks slightly different than ours, which is not surprising.