He could be tapping the equivalent of left or right to change direction slowly, or tapping forward to keep going but not at maximum speed.

: I'm not sure his fingers are even in the right places, but I don't play Halo PC so
: I'll leave that analysis to someone else.

Unlike the console games, you can reassign the keys, so they could be right. The big unrealistic thing, which I didn't notice until I was thinking about where his hands were specifically, is that he's not using a mouse. No mouse? on a PC? running a first-person shooter? in this century?

: The button-tap sounds don't seem to match his fingers too well, so I expect they've
: just been added in later. Actually he doesn't look at all sure what he's doing with his
: fingers, and he's also intermittently looking at the keyboard as if to check that he's
: pressing the right things.

nervousness, maybe

: "Who's winning?" she asks. Poor fella seems to be having a hard
: time of things, and sighs. "The bad guys" he says. But it seems
: that our Carla knows her stuff. "If you stick to the north side of
: the bunker, you can pick off the Jackals one at a time" she replies
: casually after sauntering across the screen. Hapless techie is clearly
: impressed with her all-round ability. I mean, not only is this woman a
: world expert in crisis management and repelling alien invasions, she's
: also a complete babe and knows Halo inside out! You don't get that
: combination every day.

: But hold on a minute. I don't play a lot of multiplayer - ok none at all -
: but I'm pretty sure you don't fight covies on those maps. Seems to me that
: either hotshot Carla is messing with his dizzy little head, or this is a
: major anomaly!

Halo PC came out about three years before this. One of the big features of it was the Custom Edition that let you make and edit maps, weapons, textures, visual effects, sounds, sound effects, etc., including putting AI in any map you want. If it's a modification of the map, or a custom-made one that just looks like it, and it's popular or well-known, it's plausible for a Halo fan who plays on the PC to give a pointer like that to another player.
I doubt they thought that through to that level of detail, but then they did use the name of the game and the name of an actual type of enemy... I thought there was something else too but nothing's coming to mind.

: At this point they have a bit of a heart to heart about the guy's forthcoming
: marriage. As she takes a seat, you can see on the screen that the player
: is suddenly out of the hog, even though he was cruising along a split
: second ago. In front of a base and holding a fuel rod gun, the Spartan
: moves a little, even though the guy is clearly now focused on Carla (and
: he'd be a fool not to, let's face it).

The time between the two shots is more than enough for him to get out and go a little way. I'm not sure if it's realistic for him to be in a base so soon though.

: In a short while the chat concludes and she wishes him "Good luck with
: the Jackals" as she gets up to leave. Just after he thanks her, you
: can hear the sound of a fuel rod gun firing, even though he's still not
: even back on the game yet. Huh, this cheater must have someone else
: playing for him!

or it's multiplayer, or there are AI enemies with FRGs around

: That's pretty much confirmed as we get a view of the screen, where the
: player is now anomalously wielding a rocket launcher (what happened to
: the fuel rod gun?) and moving.

It could have been his secondary weapon or he could've picked it up.

: Anyway, the view stays on the screen, and from the Spartan emblem at top
: right you can now see that this is some version of Slayer. There's no sign
: of anyone with a fuel rod gun, despite the firing sounds. You can hear
: some chain-gunning too.

same as above, enemies or allies of one kind or another

: The player advances a little then sends a rocket towards what looks like an
: empty Warthog over yonder. It's now apparent why this guy was having such
: a hard time before. He hasn't yet picked up rule number one. In order to kill
: enemies, you must first be aiming at them . Silly fool has been shooting at
: nothing all this time, so it's no wonder he was having a hard time with those Jackals.

That was probably to end the scene something that looks good-- and sounds good, the magazine cycle sound of the rocket launcher sticks out a little bit through the next cut, and I like the way it's done.

: We're not quite done yet though. A little later at 31:27, we're in another
: scene. Techie doofus is with the team's medical expert; none other than
: Data from Star Trek. Ok Brent Spiner, but c'mon that's Data dammit.

Brent Spiner is a good actor, even if Data was the role he's best remembered for and arguably his best, he deserves recognition.

: Techie's previously poor eyesight has mysteriously improved after a close
: encounter of the alien kind, and Data asks if he's noticed any other
: changes. "Halo" he replies, head in hand. Data's never heard of
: it though, so an explanation is needed. "Uhh this videogame that I
: play sometimes. Er, Doctor Caffrey gave me some pointers and they helped a
: little bit but, last night, I broke through every level. It's like my
: hand-eye coordination is off the charts."

: Blimey. Broke through every level? Was that in multiplayer then, with all
: those tricky Jackals? Good work Mr Doofus!

I'm almost certain they mean he beat every level-- which still has problems; this one isn't so defensible.

: I could be wrong, but I think the main conclusion we can draw from this is
: that if you want to get real good at Halo, it's jolly useful to get
: exposed to some weird-ass extra-terrestrial mojo. So, let that be your
: tip for the day. Oh, and I'll throw in another one for free. If you're
: playing on Blood Gulch, watch out for the Jackals!

I'll try

* yes, this means you have Halo: CE and Halo: CE, and yes, in certain circumstances it's as confusing as it looks