Act Four in The Duchess of Malfi is full of captivity-themed language and imagery. In Scene 1 we are told that,
"She seems to welcome the end of misery more than shun it."
Her treatment in the palace that used to be hers to manage tells us that she is restrained there because she is being punished. Note the choice of words (ex. "imprisonment") used by Frederico. The probablity seems to be that if she was tied up as if she were mad, it would be a self-fulfilling prophecy and she would become so. The word "restraint" is even used :
"this restraint, Like Eng;ish mastives that grow feirce with tying."
This is followed by the language of religious guilt, punishment, penitence and horror. Darkness is used to horrible effect as the Duchess realises what she has just done with the dead hand. The effect of all this is to get the audience to engage with the characters. Tragic elements are produced as they would themselves have been familiar with the terror of these religious "end of life" threats.
Later Cariola reassures her that she will want to "to shake this durance off," but her thoughts turn to snared birds, just as Bosola wants her to. Earlier, she had felt envy of a bird's ability to "choose its mate and sing for the joy of spring." He wants her to know and pine for the freedom she has lost.