MAD is used to determine irrigation amount (field capacity) applied to a zone to fill the bucket.
The equation used is
MAD * Root zone depth (RZD) * available water capacity (AWC - derived from soil type).
So, for my zone I have cool season grass (RZD = 6), MAD at default (50%), and soil type of clay loam (AWC = .2)
6 * .5 * .2 = .6 inches.
For my zone the Iro will track to .6 inches. Now, each day ET pulls this number down, and precip + irrigation pull it up.
When it hits zero we water.
MAD is used in the part of the equation which determines irrigation amount, which in turn is used for frequency and duration.
So, when MAD is set lower than 50%, let's say 30%, my irrigation amount would be .36 inches.
It will now be quicker to reach zero, hence watering frequency is increased (more often), but my duration will go down since I only need to fill .36 inches versus .6 inches.
Hope this helps.