I found this way when I brought my uncle's HTC One ( Android 4.3 with Sense 5.5 ) and he locked some apps like Gmail, Astro File Manager,AppStoreVN,... pretty cool,ha?
1.Open some apps which doesn't lock like calendar,dialer,..
2.Close that app.
3.Open locked apps and unlocked at at the same time ( first the locked app, then the unlocked app, do it quick )
4.That should open unlocked app.Press back button and...bang! Your unlocked app is open