Question about a ' dd ' command.

[elmo]

I read somewhere that these two commands are special application of  the
basic 'dd' command that will copy and display the transfer action.
 
$ dd if=/dev/zero of=/dev/null& pid=$!
$ kill -USR1 $pid; sleep 1; kill $pid

I don't understand the null& and pid=$! in the first line and I don't
understand any of the second line.  There doesn't seem to be any
explanation of these anywhere.  It appears to me that the writer used
some notations that can be confusing to other guys like me,

Is there  a  more complete  discussion of  these  commands somewhere?

Could someone please show me what the commands
would look like if the simple basic version of ' dd 'is:

      dd if =/dev/sda2 of=/dev/sdc3  bs=4096