Pass-through Vs. 301 redirects

Hi all,
I'm redesigning my website.
Most of my old URLs end in .html, and my new site was built using WordPress.
My developer has instructed me to use a "pass-through" redirect, and I wonder what impact (if at all does that have on SEO).
I read that a 301 redirect is the most preferable way to redirect from old URLs for SEO, but can't find anything on pass-throughs, and what their impact is.
Can anyone help?
Thanks!

See original post by mony49