When a twist is unwound without cutting the DNA strands or is removed by cutting the strand(s) and resealing, negative supercoils are introduced in the DNA.

From Cell and Molecular Biology -Karp

But strangely enough unwinding of DNA by helicase cause the DNA ahead of the primosome to form positive supercoils. Why is it so?

From: Molecular cell biology -Lodish

My speculations: Underwinding cause the adjacent DNA to overwind and as the DNA overwinds the associated stress developed cause the DNA to form positive supercoils (which would have happened naturally had new twists been introduced in the DNA _(Time:1:54) ).

I think removal of twist creates negative supercoils initially but when the unwinding causes too much overwinding of adjacent DNA, the strain developed cause the already negatively supercoiled DNA to form positive supercoils.