what it does is, writes 0 and 1 to the usb, all over again and again., many times.,
so its impossible for some one to recover old data from usb.
With USB-sticks and SD-cards you can't be too sure.
Flash memories are allocating their memory cells regarded parameters like health, .. of the individual memory cell.
In some circumstances there would be the possibility of saving the priv key to a cell which won't be alloced later on when you are deleting and overwriting
your usb stick. This could lead to parts of the private key still being accessible via forensic tool which are made for reading out memory cells specifically.