Download sample application - GroovyValidation.zip. This sample implements pretty simple validation rule, to demonstrate how you can use mentioned Groovy keywords:
Reserved keyword oldValue is used to implement validation logic:
Developer can specify token method expressions in validation failure message based on the same oldValue and newValue:
On runtime, if validation rule will fail, it will show current wrong value. If you will not use newValue, it will show currently in database stored value, which is not correct.