About 5" of 1/2"-wide, .5g/inch lead tape strips going up the inside of each neck piece (so ~5g total, 2.5g on each side), starting at the base of the "V" of the neck, per this photo take on 12/18/24 (click to enlarge):
What does this do for him? Primarily, it de-polarizes the frame, increases MGR/i, slightly increases swing weight and ever-so-slightly increases recoil weight, all of which together will allow Carlos to swing with a more fluid, lose wrist coupling, counteract more collision force with the ball more effortlessly, slightly increase comfort, and change the swing physics more towards the feeling of "pushing a handle" through contact (ie. more Federer) versus "pulling a hoop" through contact (ie. less Zverev), which will allow him to more easily control timing and precision, on not just serve but also ground strokes.
The dumbed-down, media-published version is here:
Carlos Alcaraz is transforming his technique ahead of the 2025 Australian Open.
www.atptour.com
Also, he/his team "say" it's only 5g, but it may very well be more than that, as the tape looks like it could be a bit wider than 1/2", and could very easily be more than .5g/inch (1g/inch, etc).