How does the ksh ENV "trick" work?

[Eric Halil]

In "The Kornshell Command and Programming Language" by Morris I. Bolsky and
David G. Korn, Prentice-Hall, 1989, ISBN 0-13-516972-0, page 78 they give:

  export FILE=$HOME/.kshrc
  # The subscript below evaluates to 0 when interactive.
  ENV='${FILE[(_$-=0)+(_=1)-_${-%%*i*}]}'

as a easy way to stop your .kshrc being evaluated when it's an not an
interactive shell.  Can someone explain in detail how this works? 

Thanks,
Eric (eric at cc.uq.oz.au)