Who offers to demonstrate that the stabbing could have happened, and what tense moment occurs?

There are only 2 acts to this play. I think you are referring to when the 2nd juror who offers to stage the stabbing. 

5th Juror gets up and begins to examine the switchblade. He explains that it would make no sense for someone to stab someone like that with a switchblade because it would require the attacker to lose precious time. Because the defendant was highly competent with a switchblade, it becomes questionable whether or not he would have made that chest wound.

7th Juror, tired of sitting through these deliberations, decides to change his vote to ‘not guilty.’