Travel by express train between German cities from only €12.90

The German national operator has launched what it is calling an auction of tickets for journeys by ICE, EC and IC trains on certain routes within Germany.
In effect the minimum price of travelling with Second Class discounted 'Super Saver' tickets has been reduced by €5 from €17.90 to only €12.90.

DB is open about the fact that this from €12.90 price is available on 'selected routes', but it doesn't specify what these routes are, so ShowMeTheJourney has spent a happy couple of hours, searching for these 'from €12.90' prices on the DB website.
It is clear from this research is that DB has evidently placed a maximum distance cap on the journeys which are available on this bargain price, because the popular routes on which the €12.90 price was fairly widely available includes:

• Berlin ↔ Hamburg
• Berlin ↔ Hannover
• Berlin ↔ Dresden
• Koln ↔ Frankfurt
• Koln ↔ Hannover
• Koln ↔ Heidelberg
• Munchen ↔ Stuttgart
• Munchen ↔ Wurzburg
• Frankfurt ↔ Stuttgart
• Frankfurt ↔ Kassel
• Frankfurt ↔ Erfurt
  ShowMeTheJourney couldn't find the €12.90 price on any journey of more than three hours; though that is very much not a 'rule' and they may well be available for longer journeys.
  Though it seems as though travelling from Berlin to Munchen/Munich or from Hamburg to Stuttgart and on other routes of the longest possible routes across the country won't be possible for €12.90.

Though on these shorter-distance routes hunting down the €12.90 price could be something of a needle-in-a-haystack scenario, so it seems they're more likely to be available if you're booking more than 4 months ahead of the travel date; tickets can booked up to 6 months for express train journeys on the DB website.
DB is also open about the fact the €12.90 tickets have been released in limited numbers; so they are inevitably likely to still be available the further ahead that a journey can be looked up.
SMTJ also spotted prices from €14.90 on certain routes and this is also lower than the typical minimum price of €17.90.

The routes on which the €12.90 price is available on this 'auction' appears to mirror the routes on which travellers aged 15-26 can now book German rail tickets from €12.90.
DB has introduced what seems to be permanent lower prices which are available to anyone in the 15-26 age group; these tickets are called 'Super Sparpreis Young'.

In effect DB has very smartly made available limited numbers of tickets to those aged 27 and over, at the same from €12.90 price, on the routes on which the 'Young' tickets are now available at this lower rate.

The general T&Cs of booking tickets at the new lower price are unchanged from the general 'Super Saver' tickets, so anyone aged 15 and over who books at the €12.90 price can take up to four children aged 6 - 14 with them at no additional charge.