When nipype decide to use the past intermediate files in working directory?

As far as I know, nipye have working directory to accelerate the repeat run.
But When nipype decide to use the past intermediate files in working directory?

Because sometimes, I changed some parameters or design matrix, but I got a same result as before. Sometimes, I got new results.

So, I am very confused about when nipype reuse the past intermediate files.

And Does nipype have any options or parameters to allow me to rewrite the intermediate files in working directory?