"Bring to boil" means a rapidly bubbling action that when you stir it it won't stop bubbling. When you have bubbles that you can stir down and they stop momentarily, it is only "simmering".

"Bring back to boil and boil for x minutes" means you bring the ingredients back up to the rapidly boiling action and continue to boil for x minutes, usually after addition of other ingredients. It is also especially important for thick things like salsa that the rapidly boiling action continue while you are stirring for the time stated (use a barbeque mitt to protect your arms from the volcanic action). Another example is for jam that you macerate overnight, bring to a boil and continue the action for x more minutes. If you just let it simmer, you will not get the gel effect of the pectin, natural or otherwise, to happen.

Also note that the boil stage may not be the same temperature as water boiling at 212F so using a thermometer is not always accurate. For example, I've found that rapidly boiling salsa gets only up to 194F and then will begin to scorch before the 10 minutes are up.

You do not have to have your stove's element on high to maintain a boil. And the BWB canner really only needs to be 180F (simmering) not 212F.

Nancy

Dave