This definition, for "seize upon" rather than just "seize," best captures the meaning in this context. To "seize upon" something often means to take exploit a weakness, rather than to physically "seize" something. ("Often," because one could say he "seized upon the rope," as in one of the other dictionary examples that crops up. But very frequently one would simply say that he "seized the rope," but that she "seized upon his fear of being alone," or "seized upon her opponent's lack of confidence," etc.)

This definition, for "seize upon" rather than just "seize," best captures the meaning in this context. To "seize upon" something often means to exploit a weakness, rather than to physically "seize" something. ("Often," because one could say he "seized upon the rope," as in one of the other dictionary examples that crops up. But very frequently one would simply say that he "seized the rope," but that she "seized upon his fear of being alone," or "seized upon her opponent's lack of confidence," etc.)