Reading with a pen in hand is a way to test what we learn because we need to restitute the concept and introduce it.
It helps to counter re-reading and the mere-exposure effect (what we want to avoid in Speed reading?) that just make us feel smarter rather than actualy becoming smarter
Is this close to Richard Feymann learning technic? (Considering a concept learned only when we can restitute it, i.e., teach it.)
Still reading with a pen in hand for writing notes is deliberate practice. It’s the same mechanism and output as having to teach a subject.