But in simultaneous translation, we need to translate concurrently with the (growing) source sentence, so we design a new prefix-to-prefix architecture to (be trained to) predict using a source prefix.
as a very simple example within the prefix-toprefix framework, we present a wait-k policy, which first wait k source words, and then translates concurrently with the rest of source sentence, i.e., the output is always k words behind the input.