Based on BOL the right answer here is wrong.
In BOL is:
There are four evaluation modes, three of which can be automated:
On demand. This mode evaluates the policy when directly specified by the user.
On change: prevent. This automated mode uses DDL triggers to prevent policy violations.
On change: log only. This automated mode uses event notification to evaluate a policy when a relevant change is made.
On schedule. This automated mode uses a SQL Server Agent job to periodically evaluate a policy.
Here is missing On Demand!!!!!!!!:w00t: