I now live in the UK and did it yesterday when driving through some loose chippings with a friend from the UK.
He asked me what I was doing and I said I'd always done it as everyone did it when I was a child in Ireland.
He thought I was mad as he'd never heard of it.
Maybe I am mad. Do other people do this or was it some piseog rural people in Ireland had 40 or 50 years ago or does it actually work in somehow absorbing the shock from the stone chipping hitting the windscreen?