How is that a dirty trick? I had that part also and asked the same question.
I don't think it's a trick exactly, but it is not nice to put people on the spot, exposing their "sin" in front of others. You know full well what will happen. Those who raised their hands will be considered bad association and will be socially ostracized by some. While that may feel perfectly routine in a high control group it is not kind or productive in a healthy group.
If The Truth is really in people's hearts then they do not need to be manipulated to follow it. Jehovah will lead them in the right direction with His loving kindness, won't He? Jehovah is omnipotent and has no need for manipulation. The Truth shall conquer all, right?
I can't say it better than this [source]:
TACTIC 4. Frequent and intense attempts are made to cause a person to re-evaluate the most central aspects of his or her experience of self and prior conduct in negative ways. Efforts are designed to destabilize and undermine the subject's basic consciousness, reality awareness, world view, emotional control, and defense mechanisms as well as getting them to reinterpret their life's history, and adopt a new version of causality.
TACTIC 5. Intense and frequent attempts are made to undermine a person's confidence in himself and his judgment, creating a sense of powerlessness.
TACTIC 6. Nonphysical punishments are used such as intense humiliation, loss of privilege, social isolation, social status changes, intense guilt, anxiety, manipulation and other techniques for creating strong aversive emotional arousals, etc.
TACTIC 7. Certain secular psychological threats [force] are used or are present: That failure to adopt the approved attitude, belief, or consequent behavior will lead to severe punishment or dire consequence, (e.g. physical or mental illness, the reappearance of a prior physical illness, drug dependence, economic collapse, social failure, divorce, disintegration, failure to find a mate, etc.).