Seeing First used is rare in the extreme in SCA fencing, at least from what I've seen. I'm pretty sure that this is due to a number of factors - it's not a natural looking or feeling guard, it takes a lot of strength and endurance, head cuts aren't typically an issue (save for C&T), and I believe it to be more useful in Fabris' fight than, say, the more upright fight of Capo Ferro or Giganti - but that may not be saying too much. If you're standing in Capo Ferro's upright and backward-leaning stance, denying your head and upper chest, taking a guard which defends those areas isn't generally going to be the most optimal action you might take.
(As a reminder, here are the visuals!)
We're going to speed through the first guard, mostly because as Fabris says, it's not that safe and it's imperfectly formed. He does note that the high placement of the sword will cause you to rely on your off hand if someone really tries to drive home a shot underneath it, unless you break measure as part of your defense. Attacking from this would be a two tempo action, and we all know how he feels about relying on those!
Looking at the second, properly formed guard though, gives us a lot more to work with. The blade is lower, pointed at your opponent, and the forte is better positioned to defend you. All good things! Fabris notes that you don't want your opponent coming in over your sword, since that's the weakest part.
Here's where we're going to take a slight tangent! Up until now, a lot of the focus on keeping your blade position strong has involved the hand position and the true edge; if you want to oppose your opponent's blade on the inside, you use Fourth, with your hand turned palm up and the true edge in your opponent's blade. Similarly, if you want to oppose your opponent's blade on the outside, you use Second, with your hand palm down and your true edge, again, in your opponent's blade. What we're seeing here (and what we'll see in future guards) is a an emphasis on the fact that the blade is stronger in the direction towards which it is pointing - and it's far easier to point the blade downwards. Additionally, in this (or any) situation, just having your blade physically on top is going to be a pretty huge advantage when it comes to displacing your opponent's weapon. (Devon Boorman's statement to me on finding the blade was something to the effect of "you can cheat a lot of the other requirements, but crossing on top is the biggest advantage you can get.")
Anyhow. Fabris says that you can just keep moving towards your opponent and find and remove his blade from your presence as you do so. You want to wound your opponent while you are over his sword, and on the outside. If he cavziones and tries to get on top of your blade, you can wound him underneath in the same way "by just lowering your body and widening your step even more, while still keeping the arm in the same position."
I can see a couple of ways this statement can be taken, and I believe that they are all accurate. If your opponent is below your blade, you can step in, lower your body, moving your forte through his blade, and strike him. If your opponent has performed a cavazione above your blade, lowering your body and passing underneath your opponent's point will keep you safe as you strike. If possible, you can pick up your opponent's blade to the outside, as he mentions earlier. You can see this concept illustrated here - on the left, we see our fencer passing cleanly underneath the opponent's blade with no contact, and on the right we see our fencer in First, with his hilt on the opponent's blade, pushing them to the outside, just as described here!
We'll close with Fabris' statement, "This guard would be just as good as any other if it were not so fatiguing to hold the arm in such a manner for a long time." Yeah, ain't that the truth. I'll be trying to do more with First, either as a guard or as something to specifically mutate my posture into.