[droffel]

The piece you are missing is called the Key Image. The key image can be thought of as a fingerprint/hash (one way function) of the true spender, and is always produced when the true spender signs the transaction. Signing the transaction with any of the decoy keys will not produce a valid key image, and therefore they are not able to spend the funds (or see their amount). This key image is also used to verify that transactions aren't being spent twice - despite not knowing which input in a ring is actually being spent, a transaction with a key image that has never been used before guarantees that it hasn't been spent in the past.