With this plugin you may increase the number of the subscribers (or any other premium group) on your board by "forcing" them to change their group (by paid system, promote system or any other) to read sent private conversations to them.
IPS4 normally could disallow per group to use conversations system but anyone from that group can't receive any conversations. By using this modification they CAN receive conversations but to read them they must upgrade their group on your board.
It's really helpfull on various boards.
select affected groups - they can receive conversations but can't read them,
enter custom "error" text - you can enter any text encouraging them to actively participate in the forum.
They can't read it in any way - e-mail notification has hidden text, the body od the conversation will be hidden on the board too - in notifications bar, on the messages list too etc.