Glossary: Uphill Efficiency / Downhill Efficiency
Uphill and downhill efficiency are similar in concept to the decoupling rate. While the decoupling rate looks at the ratio of the aerobic efficiency of the first and second half, the uphill efficiency compares the aerobic efficiency of the uphill passages with the flat ones - analogous for downhill.
For running the aerobic efficiency is based on grade adjusted pace, for cycling it is based on performance data.
Since both the grade adjusted pace and power should theoretically always be equally exhausting, no matter how steep it is, the heart rate should also be constant at constant (grade adjusted) pace/power. Therefore an uphill/downhill efficiency of 1.0 is to be expected in principle. Values below 1 can be an indication of poor technique or difficult path conditions.