When I was working out at the gym last week, I saw two guys practicing what looked like a Japanese form of karate. The teacher threw a punch at the student, the student blocked to the outside thus turning the teacher slightly sideways, in order to get a clean elbow strike into the kidney or ribs.
I didn't believe what came next. As the student struck out, he dropped his shoulder and leaned forward to make contact. Now, I understand he was a beginner, but I watched the teacher do the same thing as the student. (However, the teacher did wind up better for the strike.)
The proper way to do this technique is to keep your back straight and step in as you go for the strike. By stepping in, you have better balance. Balance is always a major factor because it allows you to react better when necessary and adjust accordingly. Also -- and equally as important -- real power in technique comes from flexibility and proper body positioning: your core, hips, shoulders and arms moving in unison.
This applies for all movements. So, the next time you practice forms, techniques or sparring, remember: Posture, Posture, Posture.