Percussion feature on electronic keyboards

I have an Optimus keyboard I bought at Radio Shack. Like most modern keyboards, it has a Percussion feature which appears as one of the “tones” available. When this tone is selected, the keys play the percussion sound indicated by an icon above each key. (The “voice bass” on the highest B natural is really funny. :D) But how would I put this to use in playing music on the keyboard? I found nothing about it in the manual.