I couldn't answer that one.

I more often then not always see someone using a upgrade voucher though when ever I am checking in.

But they do control their loads quite well. An example of this was (and this is always what happens with me) DEL-BOM I will wait list for an upgrade and loads will be wide open but leading to a few days before the flight the business cabin will fill up and show J2 D0 scenario.

I turn up at the airport and with J2 D0 still get the upgrade 80 % of the time. Now they couldn't have filled up the cabin with upgrades before hand so I am guessing they are all paid pax's in the cabin.