It seems that so few people really know how moral works in the 40k universe, so I thought I would do a quickie on it. First things first.
A unit that is falling back, automatically fails all moral checks. How is this done to someone? The most obvious is multiple tank shocks. A squad is tank shocked and breaks. It falls back 2d6. The player then sees that the unit is still within range of another vehicle, and tank shocks you again. Once again you are required to make a check, and becuase you autofail, you are required to fall back another 2d6. Certain psykers and weapons can do this to you as well during the shooting phase. Most notably, Divine Pronouncement, and Fear of Darkness (Blood Angels.). These make you make an immediate moral test of fall back immediately. When this happens, and you take 25% casualties during that phase, you will fall back again. Witch Hunters also have flamers that can do this.
Next you have modifiers to leadership, and things that just lower it. The difference is a modifier simply gives you a negative modifier to your roll. Your leadership stays the same. Stubborn units are immune to this.
Next. Assaulting a unit that is falling back. Declare the assault. The unit that is falling back must immediately make a leadership test. If it fails, the unit is wiped out, and the unit that declared the assault does not move. If they pass, they immediately regroup w/o a consolidation move, and the unit assaulting does so normally.
When falling back. The unit falling back must fall back directly towards the board edge. They move over dangerous and difficult terrain, not around it. This means its possible for them to fall back, run over a destroyed vehicle, take 25% casualties during that phase, and would then be required to run another 2d6. Any phase that you take 25% casualties you are required to make a moral check. A unit falling back must always take the shortest route towards their board edge, even if that means running forward and around units. Its only when there is no way out, and they are required to move a distance that cannot get them anywhere that they are trapped and destroyed.