The slider (grey) is an internal part that houses the capsules. The user can slide this part back and forth to select which capsule to use. Arrows on the main body indicate which capsules is going to be used. Additionally, the part snaps into the correct alignment when pushed to make sure the capsules are perfectly aligned before the lid is closed.
Underneath the slider is another part (light grey). This part pushes the slider up slightly while the lid is open. The reason for this is that the slider needs to move over the needle at the bottom. The slider has pins that slide into this lower part. The holes on these parts contain small springs that will push the slider up. When the lid is closed, a small needle at the bottom will puncture the capsule from the bottom. This will create a small hole in the capsule which then allows the coffee to flow through. See image below for more detail on this mechanism.