This is a little tweak that decreases the amount of TUs the aliens have during your first turn in Battlescape.
I have the plans of making this amount equal to the amount of TUs they normally reserve when in Patrol mode depending on their aggression stat. However, for now I had to go with one common amount for all of them, because I have no means to query the spawned unit about its aggression. So I went with 33% because: a) it is more or less average figure (they reserve 0% TUs with aggression 3+, 33% with aggression 2, 40% with aggression 1, and 50% with aggression 0 — my thanks to Meridian for providing that information); b) it corresponds to aggression 2, which is the most likely aggression level for the units you are going to encounter during the first turn.
As to the rationale for decreasing their first-turn TUs, there are two reasons: a gameplay-related one and a "realism"-related one.
Gameplay. Currently, on your first turn you either risk facing significant reaction fire, or just wait this turn out (if you don't see any enemies that can shoot you right inside your aircraft). This leads to the first turn always (with the above-mentioned limitation) being skipped. It means we might just as well give the aliens on your first turn the amount of TUs that they usually reserve. This way we get rid of: a) one routine action (clicking End of Turn); b) the element of being (un)lucky, because you can't just skip the first turn if an enemy has spawned in such a way that it can see the inside of your craft (I'm not a great fan of relying on luck).
"Realism". Of course, realism in a game is a very relative thing, and by itself it's not a reason to adjust something in the gameplay. But if there is a gameplay reason for some adjustment, then realism-related concerns can reinforce the decision. And the aliens having full TUs on your first turn do not make much sense. Did they have nothing to do until you arrived? Were they just standing there twiddling their thumbs? If a UFO has landed for its mission, they would be busy carrying out the said mission. If it has crash-landed, busy with repairing it: when your UFO is missing half a hull and a Power Source, I suppose even soldiers would have to lend a hand to engineers (not all work is gonna require qualified engineers in such a situation, there will be plenty of menial tasks). So they would have to rush out on your arrival, moving hurriedly, and barely having TUs to reserve. With the terror missions, the same: the aliens would be busy chasing civilians, civilians busy
running away from aliens stupidly walking here and there, as they do
![Smiley :)](https://openxcom.org/forum/Smileys/default/smiley.gif)
So from the "realism" point of view it also makes sense that the aliens would have only their normally reserved TUs during the first turn, not the full amount.