The easiest way of this explaining the question will be by example.
Let's say we've got 4 columns: ToDo, WIP, Testing, Done.
Two scenarios:
In sprint1 we estimate story1 to be a 5-pointer. The story stays in WIP to the end of the sprint. Burndown chart shows 0/5 points done.
We start sprint2 with that story and we move it to Done. Burndown chart shows 5/5 points done.
Another scenario:
In sprint1 we estimate story1 to be a 5-pointer. We finish WIP and move it to Testing. Burndown chart shows 0/5 points done (I suspect?).
Now we're planning sprint2 and zero the story, so it's a zero pointer (because WIP part is finished). We start sprint2 with that story and we move it to Done. Burndown chart shows 0/0 or 5/5 points done?
The question is asked because I'm not sure if when I do it I'm missing those points completely and they never appear in my burndown charts or it's keeping them in memory. I heard it is a common practice and it adds those points to the completed points of burndown chart and velocity, but it's not completely logical for me and I want to be sure how it works.