Mode Aware ARM will disable 2.4GHz if it deems there is enough coverage across the channels (based upon what it is hearing around it from valid APs). I have only turned it on twice; in both cases we reverted back due to it being too aggressive and leaving some spotty areas on 2.4. I'd recommend you leave it off unless you feel that your AP deployment is too dense and interfering with eachother. I'd prefer to turn off lower data rates if possible to decrease the cell size of each AP on the 2.4 radio.
For reference, the following summarizes Mode Aware ARM:
The first problem in mode aware ARM is to detect edge APs, i.e. APs that are in the edge of the building. This is important to ensure that edge APs are not turned off by ARM. This will automatically cause non-edge APs to be turned off if coverage is very high. The AP edge status is determined by the controller based on information received during ARM update messages from APs. This is done based on number of valid APs seen by each AP. The AP with the least number of valid APs seen is classified as an edge AP
- Converting APs to APMs
- If no active clients AND
- not an edge AP AND
- tx-power closer to arm-min than arm-max AND
- coverage is greater than acceptable on all channels on the band without itself
then transition to APM
- Converting APMs to APs
- If empty channel available (no interfering APs) OR
- zero coverage channel available
- tx-power closer to arm-min than arm-max AND
- Min covered channel has coverage less than acceptable OR
- Classified as Edge AP then transition to AP