what's the little plastic stick for that came with my kitchenaid k400?

That's the tamper. It's for pushing ingredients down toward the blades during blending, especially for thick mixtures.

Here's how to use it safely:

1. If you need to move ingredients around while blending, first press the stop button and let the blender come to a complete stop.
2. Remove only the lid center cap from the main lid.
3. Insert the tamper through the opening to stir or press contents down toward the blade.
4. Remove the tamper, then replace the lid center cap securely.
5. You can now resume blending.

Important: Never use the tamper while the blender is running. Never remove the entire lid while the blades are spinning. Always wait for a full stop.

It's very useful for breaking up frozen fruit chunks when used with the pulse function.