[r-t] Poll on consecutive blows in the same position

[Alexander Holroyd]

On Sun, 28 Dec 2014, Mark Davies wrote:

> Don writes,
>
>> If we're talking only about lead-based methods, just say so, it's
>> not hard.
>
> OK: we are just talking about lead-based methods.
>
> And will likely continue to do so for some time...

Can someone explain to me what a lead-based method is, and how it differs 
from a non-lead-based method?