The chiishi (pictured above) is one of my favourite hojo undo training tools. It may not look like much, but don't let that fool you - you can do more with this little cement stick than half a dozen pieces of training equipment at the gym.
Like many of the hojo undo and kata of Goju Ryu, the chiishi is believed to have originated from China. It's said that Kanryo Higaonna Sensei (the founding father of modern-day karate) practiced daily with the chiishi and other hojo undo implements during his 14 years of martial arts study in Fuzhou, before returning to Okinawa.
Today, the chiishi has become a relatively common training aid in Okinawa and in the honbu dojo we'd usually train with the chiishi as part of our general hojo undo sessions at least 2 - 3 times a week. As with all hojo undo, the chiishi has a standard series of techniques to work through (the chiishi kata), as well as an infinite number of variations. On average the exercises take approximately 15-20 minutes, however with variations you could easily spend hours working through techniques.
The chiishi is generally described as a tool for conditioning and strengthening the wrists and arms but it's also excellent for training blocking and striking techniques as well as the importance of synchronizing breathing with whole-body power. We were often told that it's important to practice hojo undo with the intent of improving techniques and kata. In this sense, chiishi and its focus on upper body and breathing seem to compliment sanchin kata well - which is probably why we would frequently begin sanchin training in this way.
For those that are interested in the chiishi kata, there's an excellent series of instructional pics (with the Japanese) on Sensei Helmut Leitner's website. There's also a good black and white YouTube video of Higaonna Sensei with a section on the chiishi kata (3:24-6:35).
And, if you're interested in some of the other applications and variations I've included a video of Alessandro Sensei demonstrating some advanced chiishi techniques in the honbu dojo.