Here it is:
https://arxiv.org/abs/1611.06188RNN outputs "confidence" bit which can guide computation to perform more steps to obtain more confidence in the result. Essentially, RNN asks "let me think about that some more".
But, separate ablation study found that if you just drop confidence bit altogether and allow RNN to compute some more every time (e.g., always perform 4 computations on single input for 1 output), you get same or better results without extra complexity of training.
There is also Microsoft Research's paper I can't find right now about the variable computation for image classification where there is a "confidence" bit at some of the final layers - if lower layer is cinfident enough, it's output will be used for classification, otherwise the output of that layer will be passed into more transformation of upper layers.