Reference: Fast Cavalry, Page 23.

Fast cavalry are capable of incredible feats of speed and manoeuvre, being able to make any number of turns without penalty and capable of changing their formation by any number of ranks without penalty once as they move. This enables a unit in a column to deploy to its left and right into a single line like this.

The first thing to note is that the unit has deployed as equally as possible about the front model. It would deploy about the middle model in the front if the unit were formed into a wider column. Units changing their frontage rapidly in this fashion always do so about the centre front model (the leader). This applies equally to units forming column from line, or otherwise adding or reducing ranks to lengthen or shorten a line.

The second thing to note is that the models at the rear of the column have actually moved a long way! The longer the column the further they move. This is not a problem in itself - fast cavalry are supposed to be fast and manoeuvrable after all - but it can become a problem if the column is very long indeed, say twenty models for example.

For this reason, when deploying from a column into a line, models must be within a move of the position they wish to occupy in the front rank. If they cannot reach the front rank they are automatically placed in a second or rear rank. Models may actually move further than their move in order to reach a rear rank, but if they do so the frontage of the unit will be reduced as a result.

The third point is that models moving from the rear of the column might find their most direct path blocked by difficult terrain, obstacles or other troops. In this case models must be able to trace a clear path to their new position without crossing the difficult terrain/obstacle/troops, and they may not travel further than a move to do so as already discussed. The next diagram shows how this happens when a column rides past and behind an enemy unit and then deploys into a line.

As a general rule, if models at the rear of a formation cannot trace a path to their intended position without moving into difficult terrain, crossing an obstacle, or moving through other troops then the unit cannot redeploy as intended.