Hara & Koshi

Extracted from the below web

http://www.shuriway.co.uk/hara.html

abdomen & hip

'Te de tsukuna, hara de tsuke; Ashi de keruna, koshi de kere'.

(Do not punch with your arm, punch with your 'HARA';
do not kick with your leg, kick with your 'KOSHI').

'HARA' generally means 'abdomen', but in martial arts it means more specifically the lower abdomen. This is where your center of gravity should be. Without a stable center of gravity you cannot keep good balance and perform techniques with weight behind them. To be able to keep one's center of gravity at the lower abdomen, one has to learn abdominal breathing, which involves up and down movement of the diaphragm.

Refer to the above link for more information .. a good resource web