it uses a simple three zone method which is outlined below and is based on your heart rate zones during exercise.

Zone 1 is up to 75% of maximum heart rate (HRmax), Zone 2 75 to 85% HRmax and Zone 3 is above 85% HRmax. 

Once you have split your session into the three zones (a session may only be in one zone, for example a long easy session would remain in Zone 1 throughout) you can then use the TRIMP calculator to work out your recovery strategy for you.

 Also if performances have been off-par or another session is due in 4-6 hours then you may need to also upgrade the need to accelerate glycogen restoration in that key 4-6 hour period that recovery drinks can help over normal foods.