So when a person like Zimmerman refuses to take the stand in his own defense

[AngryAmish]

64. This is the thing: juries ignore the instructions

Whether they admit it or not, each of the jurors have their mind made up already. They usually make it up during opening statements. And once someone makes up their mind they filter all the evidence through their particular preconceived notion. They ignore evidence that is against their prejudice while seizing on anything to support their versions of events. There is a lot of research that backs this up.

So do people change their votes in the jury room? Sure, to get along with the group. But if the jurors are getting along they will reach a consensus. It usually is pretty quick but sometimes the jury stays out because they don't want people to think they came to a conclusion too quick. But if they go out on a Friday afternoon, expect a quick verdict.

The point of closing and jury instructions is to give people on your side in the jury room something to argue to other jurors - and that only works if they are already with your side. SO if jurors 1-4 are for guilty and 5 and 6 not guilty, then juror 1 will pick up the jury instruction and say "well the law says X". Jurors 5 and 6, now knowing that the group wants guilty, will change their mind and the jury instruction allows them the fig leaf to say their mind is changed - even if in their heart of hearts it is not changed.

Also, it is not entirely unknown to allow an improper instruction to get in to give yourself reversible error in case things break bad in the jury room.

Every trial lawyer hates jury instructions - they are a pain in the arse.