"The top position (1, DISCHARGE, SLOW) is the onboard Battery Conditioner; which drains the cells of all residual charge down to 1V before automatically beginning a full recharge cycle. This process prevents "sulfation" from building up in the battery, (which is the chemical substance that causes capacity reduction.) Using this feature occasionally helps your cells maintain top performance for throughout their entire lifespan. (Its also helpful for reconditioning some previously depleted batteries.)
The bottom position, (2, CHARGE, FAST,) is the normal charge mode, and using it recharges batteries as quickly as possible. Use this when you want fastest charging performance.
Our personal recommendation is to leave NiMH batteries in your device until they are depleted, and then recharge them with the Battery Conditioner to maximize their charge capacity. It is not necessary to discharge batteries only needing to be "topped off". Neither is it necessary to discharge them if they are not fully depleted. Running down batteries in whatever device they are in is the best method for draining them naturally. The Battery Conditioner will help keep them healthy and in top condition at all times. "