How can I get a partial refund from Booking.com / Hotels.com?

I've seen people offering services to get a partial refund of around 50% - 70% from Booking.com , Hotels.com and some other websites as well.
The thing is they are offering the service only if it's been 5 days BEFORE you are visiting the hotel. Do you have any ideas how it works? How can I do it by myself?
What should I tell them?

I'd be highly interested in this as well! Or any other Hotel SE method.

Another method is to just email the hotel or B&B , etc. And email or call them that you want to make a reservation.
Always ask for the best price compared to booking.com ( most of the time i get 20-25% discount on the offered price on booking.com)
Its not the method that youre looking for but its something.....

I really have no idea how It could works. Maybe someone could share the information with us..