Although the grand jury has convened some time prior to this date, a surprising twist occured this week when the target officer, Darren Wilson, took the stand to testify before them. This very unusual step took place just yesterday. Apparently he testified for four hours. The reason a potential defendant does not take the stand at this stage of the proceedings is that he bears a complete and absolute right against self-incrimination. Meaning - he doesn't have to testify. No one can force him to testify. Tactically, the defendant is in much better position by not testifying at this proceeding. His testimony is under oath and transcribed. So he cannot change it later and must be fully prepped prior to answering questions at a time when all the facts are not even known. By choosing to testify he "locks" himself into his version of events and may make incriminating statements that harm him.
In this case it is assumed that Darren Wilson will claim self-defense or justification for his actions. That defense is an affirmative defense. That means that he cannot be held liable should the State prove their case and he proves he had to act to prevent imminent serious physical harm or death to himself. Normally a grand jury will not consider an affirmative defense during their deliberations. The grand jury's sole job is to determine whether probable cause exists to force the defendant to answer the charge. Given the manner in which grand jurys proceed it is odd that he chose to testify in this case.
Prosecutors have a bully pulpit in grand jury proceedings. They can ask the jury to issue a "true bill" (charge) or make no recommendation. They are routinely asked questions by the jurors and can give input to them when asked. They can make recommendations about fact witnesses and how to compel them to testify before the grand jury if they desire. In this case it will be interesting to see how the prosecutor acted during this process. The transcripts are being released to the public for some reason. Normally the transcripts are held secretly and not divulged to any party outside the proceedings.