During the scan, Fingerprint Lock gives you a acoustic and haptic feedback, like a real Fingerprintreader. If you don't like, you can disable this in options. For the settings-screen perform a long click on the title of the app.
Click short on the title for changing between "allow access" and "deny" on next scan. Your friends didn't know, so they will think you've built a real biometric fingerprint reader in the touch screen of your mobile. Only you know there is a trick for unlock...
Because this app is just a simulation, your tablet didn't need a real sensor. Only a few manufractures like Samsung have a build in fingerprintsensor. This App can be used with every android-device.
Note: Fingerprint Lock is explezit marked as fun app! Do not use it for securety. Protect you device allways with an password or pattern.