To get a body of warriors to move to either flank is no small challenge, especially if you want to maintain a viable fighting formation (as you do). To represent this, a unit that moves sideways moves at half rate, i.e. it counts the distance moved as being double what it actually is.
Units may not mix forward, backward and/or sideways movement as part of the same move. Remember that units can only wheel when moving forwards.
For example, a unit of Elves (Move 5) would be able to move 5" forwards or 3" (2.5" rounded up) backwards or to the side.