The eraser mold is suitable for simple part but not high detail. You just drop it in the boil water and wait until it melt, then you can use it like modelling clay.
The first link is easy, he want to copy detail up part for his Zaku. Then he made Lego block and put original piece at the center. He boil eraser and press it in Lego block over and over again to make sure eraser fill in that block and no space left, wait for cool down, then he get mold for resin cast.
This guy use it for copy Oorbellen face, but he use epoxy putty to cast instead of resin.
this guy use hot glue gun to melt eraser into jell and shoot it on original piece to make mold. I think this trick will get better detail than boil it to clay state. And when you want to recycle you just boil it in water again, and make it into stick form for hot glue gun. http://www.fg-site.net/archives/post_old/156373
But if you want to copy high detail or mass production, I prefer silicone than eraser