It's all thanks to the The Grey Book which forces you to lay down lights on the grey paper page first.
When it works well it helps to short-cut the dreaded 'pencil first' tendency that tries to sneak in from time to time. The looser the initial white is - the better the sketch turns out - and the pencil doesn't even get a look in.
The old Victorian houses in Albany Road, Ponsonby are perfect for trialing the technique and it makes the decorative white fretwork stand out nicely.