The 20 Minute High Percentage of Maximum Minute Power Test is a test for experienced cyclists that is conducted at 75-80% of maximum minute power and measures improvement in sustaining a high percentage of maximal aerobic power. It is a progress check following on from the 3 Minute Aerobic Test.
Regular progress checks are very helpful to track the effectiveness of your training plan in which improvement is usually shown by a progressively reducing heart rate for the same cadence/power output.