Double-tap

Double-tap is a theoretical keyword action which has been discussed by R&D for Time Spiral, but has never been printed. It would require to have a new symbol the same way tap and untap do.

Description
A permanent with the double-tap symbol (as a cost for an activated ability) could be tapped twice within one turn. First time you tapped it normally, tapping it ninety degrees. You could then tap it again, turning it essentially upside down. Creatures only untapped one rotation during the untap step, so if you double tapped it, it would take two turns to untap fully.

Obviously, this mechanic had to use abilities where there is an advantage to tapping it twice on the same turn. But while novel, it proved to be hard to design to and the play pattern ended up being boring players tapped twice the first turn and then just tapped it once for the rest of the game.

The exert mechanic that debuted in Amonkhet is very similar to this mechanic, allowing creatures to spend an extra turn tapped after they attack for additional benefits.