In Act 3 of Twelve Angry Men, what does juror three do to eight that causes the others to tell him to look out?

There is constant bickering and a sense of hatred between
jurors three and eight throughout all three of the acts.  In this particular section
that you are referring to, the jurors decide that it might be a good idea to recreate
the scene of the crime to see if it would have been possible for the boy to have stabbed
his father in the fashion that he was stabbed, since he was known to be an experienced
knife fighter.  Juror three volunteers to be the boy stabbing and juror eight volunteers
to be the father that is being stabbed.  At this point, juror three pretends to stab but
brings the knife so close to juror three's chest that the men actually think that he was
close to stabbing him -- which is why they yell out, "Look out."