Annotations which stick to the slide even when moving through pauses while presenting

I have seen a presentation where the presenter made annotations on their slides using a tablet while gradually uncovering blocks in the frame. The annotations made before uncovering a block stayed after the uncover for the entire duration of the slide/frame. They disappeared when moving on to the next slide. So probably no new pdf pages were created for the uncovers/pauses. How is it possible to do that and what softwares do I need?