half smash/stick smash is hit square/cleanly and virtually only uses pronation to generate speed. A fast drop is... a fast drop. You can do a fast drop by using the same technique as above, just a bit more gentle. You can also do a fast drop by slicing the shuttle and using high racket speed. If done correctly you have a drop shot with moderate speed, and high shuttle spin. The high shuttle spin will bring the shuttle down steeper. The distinction is a bit more vague than perhaps I have proposed, others may have different opinions...
Smash trajectory is relatively straight. Drop trajectory is more curved hence the name. Fast drop curves down while slow drop curves up first.
