Animating on twos
Also said as On ones, On twos, Limited animation, Held frames
Showing each drawing for two frames of a 24-frame second, so there are 12 new drawings a second. "On ones" means a new drawing every frame, and "on threes" every third. The holds are a choice, not a slowdown.
It halves the drawing work, and it can look better: snappier and more graphic, where smooth inbetweens might turn to mush. It also separates the character's rhythm from the game's frame rate.
Say it
“Animate the character on twos at 24, so 12 new poses a second, with attacks on ones for the three frames around contact. The game still runs at 60.”
Where you’ve seen it
Studio MDHR has said Cuphead was drawn on ones, 24 drawings a second, for extra fluidity. Guilty Gear Xrd goes the other way on purpose, using limited animation on 3D models so that they feel hand-drawn.
Not to be confused with low frame rate
A low frame rate makes the whole game choppy and slower to respond. Animating on twos leaves the game running at full speed and holds only the drawings.