how do i use the tamper thing that came with my kitchenaid k400 blender?

You use it to push ingredients down towards the blades while blending, especially for thick mixtures.

1. First, stop the blender and press the stop button. Let it 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 the contents down.
4. Remove the tamper, place the Lid Center Cap back on securely, and then you can resume blending.

If that didn't work, make sure you're not trying to use the tamper while the blender is running or before it has fully stopped. That's a safety issue and can damage the coupler.

Heads up: Never remove the entire jar lid or the jar itself while the blender is spinning. Always unplug it before removing the jar completely.