The discount is as follows:
Any secondary line on a family share that either performs an ELIGIBLE upgrade from a non-smartphone to a smartphone, or adds a new line to the account that includes a smartphone purchase for use on the new line (the $29.99 data plan only, not the new $15 one), the $9.99 monthly fee for the voice minutes is dropped...both of the above require a two year agreement.
Over a 24 month period that equates to a savings of $240 for each line that participates in said promo.
So instead of that secondary line being $9.99 for the sharing of minutes + $29.99 for the unlimited data = $40/month, said line will now be just $29.99/monthly.
In case there has been any confusion, I hope this clears things up. TBV