Sidney asks Clifford to act out parts of the second act including violent struggle, then reveals the struggle was to enable him to have evidence to show why he needed to kill Clifford and that he plans to burn any remnants of "Deathtrap".
The struggle was to enable him to have evidence to show why he needed to kill Clifford and that he plans to burn any remnants of "Deathtrap". This is what Sidney asks Clifford to act out parts of the second act including violent struggle, then reveals.