Reason behind formation of positive supercoils during DNA replication/ transcription
It's hard to explain in text, so here's a video:

https://www.youtube.com/watch?v=J4YlcD59-yw

Imagine the shoelaces are two DNA strands in a double helix. They are topologically constrained at each end. As the pen (helicase) moves through the helix, it creates overwound DNA in front of it and underwound DNA behind it.