How to do icing swirl effect (a bit like marbling)?

I can't be sure, but it looks like there are two different things going on here.

For the ice cream cake, notice that there is no white chocolate in the bottom swirl (the white there is a reflection of the light). So I expect they are using an implement (quite possibly the tip of a spoon) and swirling it from the dark side to the light side. Then they lift the spoon and let it trail across and then smear from dark to light again, leaving behind some of the white on each successive row.

For the rectangular cake, I think they've iced the entire thing in the light color, then added a thin layer of dark to half of the cake. Using a two pronged tool, they can then smear from dark to light and reveal the lighter color on the dark side before trailing the dark across the white side.