Chemo agan after 2 years

My dad has PC with bone mets and he's had it since 2005. When he was diagnosed he got chemo in combination with hormone treatment and it worked for several years. Next time he had chemo was october 2011 until february 2012 when he stopped the treatment. In august 2012 the PSA was back where it was before chemo.
Then he's been on Zytiga from october 2012 until now.
His PSA has started rising again and there was a small change on the MRI pictures since last time, so they are starting chemo again now.
How come that chemo works again and again when there are a few years between the treatments? Does the body "reset" in some way?

I have the same question about medicines in general. For example Zytiga. Now the effect of Zytiga is over, but will it might work again in a few years?

A medication will "work" on a certain population of cancer cells, let´s say, for the sake of making a simple illustration, on 95%. 5% it will not touch (some call this a cancer stem cell). When 95% of cell population dies, the other 5% will take over, introducing into the population a cell that is already resistent to the medication. That is why usually medications work only once, before the resistent cells introduce their phenotype into the population.

If 95% of cells did not die the first time, or the 5% of cells who survived give rise to offspring that have retained some of the characteristics of the original cell, the medication may work second, third, fourth time. This is the case, for example with intermittent hormone therapy, where after each cycle ON, the PSA drops back to 0. There are also other theories on why a medication will keep working, but most important thing is that chemo is working for your father. Quite inspiring, greetings to you both!