ÖBB is sending to Newsletter readers that have a ÖBB Account an email with a coupon with 4 reservations for free for trains with optional reservation of ÖBB. The coupon is automatically linked with your ÖBB account.
I tried now to book also DB trains, and it works (I tried till to the “paying” site). I don’t know if everybody with ÖBB account gets this or only people which have also the newsletter.
This free reservation works only for trains with optional reservation system, and in the text it says only ÖBB trains.
You have to book a reservation till 11th of September to use this free reservations.