Sandwyrm here. I've cleaned up the fall back rules a bit based on feedback.
Although moving your models around seems simple, moving around well is one of the hardest things to learn in the game. It’s not enough to just close with the enemy. You also have to hug cover and time your advances so that you can hit your foe in force at his weakest point.
These rules will cover how to move units made up of infantry. The rules for moving Vehicles will be covered later.
All Models Move In Units
All models in the game will belong to, and move within, a unit. Even if a particular unit consists of only one model.
Each unit's leader has a Command Radius, equal in inches, to double the unit's Leadership (Ld) characteristic.
Every model in a unit must end it's move with it's base completely within the Command Radius of the unit's leader. This radius is measured from the edge of the leader's base to the furthest edge of every other model in the unit.
Leaders Always Move First
The Leader of a unit is always the first model to move. If a unit's leader has been killed, then the unit itself may not move at all, although it may still Shoot, or perform other actions that do not require it to move.
The Leader’s Move Is Final
Once you have moved the unit's Leader, the Leader model may not be repositioned once you have let go of the model. So choose the position of this model carefully! You may reposition and tweak the final positions of the other models in the unit around the leader, but the leader model itself may not be relocated without your opponent's permission.
Special Rule: Swarm
A unit with the 'Swarm' special rule has no predefined leader. Instead, the unit's leader is chosen by it's owning player from amongst all surviving models at the beginning of the unit's movement for that turn. Place a marker or a die next to this model. It will be the unit's leader for the remainder of that turn.
Special Rule: Battle Tactics
A unit with the 'Battle Tactics' rule has been fully briefed on the details of the engagement and will know what to do even if the unit's original leader is killed. If the unit's leader is slain, make note of which model in the unit is closest to the leader and then replace it with the original leader model.
The new leader retains all of the equipment and abilities of the original leader and loses any special weapons or equipment that was carried by the model which was replaced.
To move a unit, you will first select the unit's leader model and move it a certain number of inches, based on the movement order given. You will then place the unit's other models anywhere that lies fully within the Command Radius of the leader model.
How far the leader, and the unit as a whole, is able to move will depend on the type of action that you have selected.
The 2-Inch Standoff Rule
Regardless of the type of move selected, no model in a unit may be placed any closer than 2" to any enemy model; unless the unit's leader has first initiated an assault.
Moving Into Assault
Normally, no member of a unit may be moved or placed within 2” of any enemy model. The only exception to this rule is when you wish to initiate an Assault. This allows your unit to engage in hand-to-hand combat with one or more enemy units.
To initiate an Assault, a unit's leader must perform an Advance, Shooting Advance, or an At-The-Double move that brings it into direct base-to-base contact with at least one model in an enemy unit.
Once an assault has been initiated in this way, the rest of the unit may be placed within 2” of enemy models, or even in base-to-base contact with them. Models which are placed in base-to-base contact with the enemy may strike them with their close-combat weapons in the Assault Phase. While the models in an assaulting unit which are not placed in base-to-base contact may only lend supporting fire to their comrades.
The precise details of how close-combat works will be covered in the rules for the Assault Phase.
When performing a Fall Back action, a unit’s leader must move directly towards the controlling player’s table edge at double their normal move. For infantry, this will normally be 12”.
A unit which Falls Back is throwing caution to the wind in an effort to get to safety quickly. If a unit is shot at by the enemy while falling back, they will suffer a -1 Ev penalty.
Should a unit be forced to fall back as a result of enemy fire, or from losing an assault, this Ev penalty will replace any other Ev penalties the unit may already be subject to.
For example, a unit which moved At The Double in it’s previous action phase would not gain and additional -1 Evasion penalty for falling back due to enemy suppression.
Stay On The Table!
A unit which is falling back will not run off of the table. If the leader’s movement would take them past the edge of the table, simply place the leader model so that it’s base touches the table edge before deploying the rest of the unit around it as normal.
Note that a unit which is falling back will only be destroyed if they fail their Ld check at the beginning of the controlling player’s next turn during the Starting Phase.