To be frank its Jiang Shi not chiang shih …
Moving corpse are divided into 2 types of categories
1) Jiang Shi which is normally buried for a long time or which is dead due to loss of blood.
2)Xing Shi which is dead but able to walk like normal people but is kinda slow.
Jiang Shi meaning for stiff corpse are stiff of course, its believe that if it consume blood, and the blood will continue to pump through the body , and the Jiang Shi would be able to move a little bit better not as stiff as before for a short period.
The time is directly affected to the amount of blood consume.When the blood drys up, the corpse will starts to get stiff.After long period without having blood,it will starts to dry up and would start to rot causing it to be unable to move again unless blood is drip in to the body causing it to get away from stiffness.

I agreed with Austin but just to add on. Why Jiang Shi is called Jiang shi? Actually it’s origin came from the Mao Mountain of China’s Jiang Xi Province (Yep the Jiang in the first name of the province). It’s actually depicted Corpses from Jiang Xi Province. In that province there is a special job known as corpse deliverer. This occupation came from the long way dated almost 1500 b.c. The deliverers from the ancient China times found out a way to deliverer corpse to it’s families by using a biological and gravitational method. Thus “Jiang Xi Corpses Chasing” became a popular term. Later in those old Hong Kong movies under the influence of late actor Mr Lim Zhen Ying (original name was Lim Geng Bao), Jiang Zhi(s) evolved to what you see as of today.