This expression is to change the framerate of a single property of keyframed animation.
I generally use it to drop the framerate of a single layers animation to 12fps inside of a 24fps comp.
It acts similarly to the Poserize Time effect, but works for keyframes without having to precomp.
You apply it to whatever property you'd like at a lower framerate.
Change the 2 to however many frames you want to be held.
if(timeToFrames() % 2 > 0){value}else{valueAtTime(time-thisComp.frameDuration)}