In this movement, you are starting with the stick in your strong hand. You want the hip of the same side holding the stick back. Keep your shoulders down and relaxed. Keep your off hand up to protect yourself.
As your start this strike, make sure your hips, shoulders, and offhand are all moving together. Remain relaxed. Keep the elbow of the striking hand in. You want the strike to be in a straight line. As the strike moves forward towards the target, your non-striking side moves back. Think of it as a push/pull motion.
The key to delivering this strike is to stay relaxed all the way up to the point just before coming into contact with the target. The striking hand should have a relaxed hold on the stick and only tighten up around the stick at the last moment before impact. As you are making impact, your body should have the feeling of driving through the target. The striking arm should be straight but do not lock your elbow.