funnily enough I was thinking of trying this exact same thing this morning - I stayed with a friend last week with a tumble dryer and put my nappies in as I needed to get them dry quickly as I was running out. I only put the heat on low as I was scared of wrecking them, so they ended up still being wet and cool after 20 mins (her tumble dryer was a bit poo!), so they were still dried on the line in the end, but they were so much softer I couldn't believe it, so I figured it must have been the friction effect. I wondered if I could replicate it in the washing machine - will definitely give it a go. I've got some eco-balls, I'm sure I can use those to try with