Here's where the magic happens.
I wouldn't say this is where power comes from though. But I would say that this is the motion where power - and accuracy - is channeled through.
It sure gives you access to "easy" power though and there is off course more to it then just this (trophy, racket drop, coil, lift,...) but without this - forget it.
When I teach the serve, the above is basically what I start with. As in: nothing else. Starting position is literally frame one... racket on edge and string bed point left. Toss ball, bring racket up, pronate and end at frame 3. Literally end... no swinging through. So that the student can literally see and feel that the frame is fully turned with hitting string side pointing fully right.
And even doing just that, you can actually get pretty decent power going already if done correctly.
Then adding in trophy, coil, racket drop and lift from legs adds loads extra racket acceleration and power gets multiplied.
A cue I often use with students, mostly with succes, is also to not consciously let the racket drop. But rather, to throw the hitting elbow up and forwards from the trophy pose while keeping the forearm / wrist nice and relaxed so it just dangles along for the ride.
If you do that, your racket drops behind you automatically. And then it becomes a "simple" 1-2 motion from trophy onwards.
1: throw elbow up and forward
2: let racket follow up and pronate / ESR