Essentially, going really slow (like really slow) through a portal causes it to ignore the portal entirely.
To make it happen, set the mouse speed really slow so and hold W up against a wall that has a portal at it, and go as slow as possible into the portal, and eventually it will skip the portal when you get to the center point of it.

Can you show a video about it?

Yes here's what I found with it, only way I can think of it being bad is when you are not meant to be able to access behind the portal and then get stuck there:
https://imgur.com/a/1JsJKXA

Possibly related to some floating point inaccuracy