Exporting variable contextual alternates font

I’ve been trying to do as you suggested and draw new glyphs using a centre line. Where do I apply a stroke thickness?

I’ve tried putting a value in the h and w boxes, but that seems to offset the line.

I’m sorry to have to ask such a stupid question, but I’ve been through the user guide and there’s no mention of centre strokes or stroke weight in it that I can see (I’m assuming this is maybe a new feature?).