What kind of striking method does jiujitsu use

It depends on the style of Jujitsu.

Old school BJJ uses open hand palm strikes, elbows, knees and basic kicks primarily. Most BJJ fighters today that compete in MMA will also spend a good amount of time boxing and Thai boxing.

Do they chamber their strikes and tense their bodies
upon impact like in karate?

Not usually.

I think the more rigid, tense, snappy movements associated with modern Karate were a result of the influence of the original Chinese and Okinawan systems, and were not common in indigenous Japanese martial arts.

The Japanese didn't worry about striking. Only grappling and using weapons.

Using weapons is striking. duhh!!!!

A few of the old Ju Jutsu Ryu emphasized striking techniques (Atemi Waza).

Most systems, even the grappling dominate systems, usually included some striking techniques (including Kodokan Judo).

When you compare Japanese striking (empty handed)methods with, Sumo, weapons systems, or the vast array of grappling focused styles, it's pretty one sided.

There certainly is striking in Japanese jujutsu, just as there is grappling in Okinawan karate. The differences stem from the overall strategy of the arts. If you focusing on the grappling elements then you will spend less time on striking and the striking you do will be based on the same general body mechanics as the grappling and be used to supplement and compliment the grappling. So in jujutsu you will mostly see: hammer fists, palm heel strikes, thumb strikes, finger pokes, front knees, and stomping kicks. But you will only see those techniques that complement the grappling.