Less popular alternatives include adding layers to the connections from input to the hidden state, between hidden states, or from the hidden state to the output. These designs employ skip connections to avoid a situation where the shortest path between time steps increases and training becomes more difficult.
This is from page 597 of the book Machine Learning for Algorithmic Trading. And this chapter is about RNN.
I don't quite understand what does 'These designs employ skip connections to avoid a situation where the shortest path between time steps increases' means. I haven't seen any materials about how to use skip connections in RNN.