Something to do with water displacement - weighing water in a large bowl on a deep tray, then putting a head in the water and weighing the water displaced into the tray and subtracting the amount. I'm sure someone else can provide a more scientific explanation. I've never tried it and don't intend to!
I don't think that putting heads in buckets of water is likely to be at all successful in determining the heads weight unless it's density is also known.
Perhaps one of the winners could post the method on receipt of the book.
I didn't bother because they were short of shillings for the meter for a while and I gave up in despair but for those made of stronger stuff ....Well done !
Though I'm not too sure about the fact tall men are more successful than short men .......! My husband is shorter than me and I am rather tall ! He's been a great success for the last 35 years .