The split step allows you to make a fast start, while still covering different shots from your opponent.
A fast start is essential
In most situations, the distance that you are moving to the shuttle is actually quite short. But you don’t have much time to cover that distance, and you need to ready to move in different directions (because your opponent can play different shots).
So in general, badminton requires short, fast movements with quick changes of direction.
Your speed on court is not determined by how fast you can run, because you never take more than a few steps before stopping and changing direction.
What really matters is how quickly you can start moving, once you see where the shuttle is going. That is why you need a good split step.
How to do a split step
The split step involves widening your base (moving your feet farther apart) and lowering your posture, at the same time, in one quick movement.
As you complete the split step, immediately push off towards the shuttle.
But doesn’t that take longer?
Often players are sceptical about the benefits of a split step, because it seems like you’re adding an extra movement, and you might think that would take longer.
But a split step is absolutely essential. Without it, you are not ready to move quickly in different directions. This can be demonstrated by comparing movement under pressure, with and without a split step. Once you see the difference, it’s obvious why a split step is needed.
It’s not always exactly the same split
The split step isn’t exactly the same every time, even though it’s often taught that way.
Depending on what’s happening in the rally, your feet will need to be
lined up in different directions as you split. And often the previous movement blends into the split step, so it may not be obvious to someone watching.
But these are details. The main point is that a good split step makes a huge difference to your movement. It’s something you need to do almost all the time.