Faye's like an untamed colt, wild and free.
But she's not really free, not in this new world in which she finds herself. In her own time, she would have been, free to travel, free to explore, free to be. In this new world, she is imprisoned by the invisible fences that surround her, chains that are as strong as the strongest iron, and as hard as she tries, she cannot break free.
When she first wheedled her way onto the ship, she thought that these people would help her to break free of the chains of this place and time, and for a while it was true. But then she got to know them, and then it didn't matter when or where she was; once they knew the real you, you were chained to them and they to you, whether or not you liked it. The chains were like roots, and the longer she remained, the deeper the roots implanted. And when she looks at him …
Spike is the wild stallion chained to this world, yearning to be free, haunted by the chains that bind him here. He looks at Faye and sees the Romany that she has named herself; when he sees her struggle to free herself from the invisible chains that bind her, he wants to hold her in his arms to reassure her that they do not exist, that the only chains are those that hold her to him, but doing so would reveal to her the depth of his feelings for her, and that is something that she is not supposed to know. So he watches her from under his eyes, ready to pull her back if she breaks free.
And when she thought of him, she questioned whether she really wanted to break those chains.
In the end, he broke them for her.