Well I opened my Metabo BS18. This is what it looks like inside:
This is the trigger itself:
The trigger is built something like this:
You have a piece of hollow plastic wedged inside another tube-like piece of plastic with a spring system inside. If there are any lateral forces involved it will create a lot of friction. If pressing straight down it works pretty smoothly (Not as smooth as the Bosch though). I added some lube to it (the gap seems too narrow for grease, I might try another time) but it didn't improve a lot.
The real issue seems to be that the two triggers are offset to the right side by about 8mm. So if you press straight down on the middle of the trigger you are adding lateral forces. The trick is to press the trigger down and with both fingers pull slightly to the right or press on the right side of the trigger. After a little practice it was more a combination of two but I can smoothly press the trigger all the way down. The lube helped a little but I could have done it without.
So it turned out to be really poor design by Metabo and its sad that I had to open the drill and study the trigger to learn how to use it. You actually need to learn how you wrap your fingers around the trigger and remember where to put them everytime you take this drill in your hand. It takes a little practice. Maybe they should have put this into the instruction manual :) I will certainly notify them of this.
This is a recording of me doing it wrong first and then "correctly": https://www.youtube.com/watch?v=OycFe77rEvE
EDIT: I went to the store yesterday - rant: I tested out a variety of different Drills. The BS18 from Metabo indeed has a terrible incredibly rough trigger. The Metabo 10.8V BS had a very good trigger. The 10.8V SB seemed to have a slight delay between pressing the trigger and the drill actually starting which felt very sluggish. I tested some Makita, Bosch and Einhell as well. The triggers were anywhere between perfect and somewhat rough. But none, not even the cheapest were as bad as the BS18. However the BS 18 isn't even that bad once you learn how to use it, it requires a different grip and then you're fine. Perhaps the people that tested it at Metabo hat strange hands. However none of the triggers got even close to how good the trigger is on the Green Bosch PSB 18 Li-2. It's perfectly smooth, and it seems like its the only drill to have logarithmic progression. It's an 18V but I love this drill for small screws because of the insane fine control at slow speeds and also the center of weight that is in front of your hand unlike other drills. I read many reviews of the PSB 18 and none mentioned the different weight balance (which can cause the drill to tip over more easily, but reduces handshake) and the logarithmic trigger. Too bad most people don't look into these things in detail. Bosch green innovations are pretty awesome at times. The trigger is something like this: