The focus of Serrada footwork is the male triangle, staying on the centerline. Though Spanish influence on FMA is often credited, this seems more akin to Italian fencing, which learned to use linear footwork to eventually counter the circular Spanish style. We do have evasion to the outside, but that is secondary to controlling centerline, so orientation will return to the most direct line of offense/defense as soon as possible.
As we work centerline, we face the direction of attack. If it comes from our left, we face left, from the right, we face that direction. In our basics, we work this with the concept of front hand, front foot, so facing left leads with the right and vice versa. Footwork alignment is like modern sport fencing, front foot/knee pointing towards the opponent, heel of the rear foot, which is perpendicular, on the same line. If you were to close your eyes and lift the front foot, your drop should be straight towards the opponent. This alignment allows us to cut the lines of an opponent's attack. It also protects the low line by keeping the angle of the legs closed against groin kicks. If you face the "wrong" way, ie. facing right with right lead, that target tends to be wide open, which isn't so bad at longer ranges but a tempting target at medium or close range.
Our foot switch is called "papeet" which Angel Cabales defined as "chicken step" (what dialect, I don't know). Many folks nowadays refer to it as the "replacement step". The point (literally) of this step is to hold the ground on which we stand, controlling the apex of the male triangle. Thus we step up with the rear foot, feet together, then back with the opposite foot (or the same one in a "false replacement", which allows rapid readjustment for alignment or balance, or to confuse an opponent as to our intentions). This is different from the chicken step I've seen in other styles, which maintains balance on the rear foot and looks, to me, more like how a chicken steps.
This isn't to say we don't move off a spot. Angel used to say we know how to move already, since we weren't born where we are now, but the goal of holding our ground is to defend a doorway, hallway, etc. where there is little room to move or strategically we cannot allow an opponent to pass. Also, at more advanced levels the front hand/front foot alignment is not absolute, allowing us to throw a right forehand with a left lead, for instance. However, we tend to stay close to the basics as they are fundamentally sound for our system.
Holding our ground means we force the opponent to come to us, allowing us to use his range and timing without showing ours first. A common error new students make in learning papeet is holding ground with the rear foot, stepping in and out with the lead, the "other" chicken step. This changes our range according to lead, and if an opponent advances as we step back, we lose ground we cannot necessarily recover. An advantage of forward replacement is it develops centripetal force to generate momentum for power in a very short space.