From landscape to a tall frame
A photo shot wide won't fit a 4:5 post or a 9:16 story without either cutting the sides or leaving ugly bars. Heightening adds real picture above and below instead, so the whole scene stays and the frame becomes tall.
The soft blurred backdrop is the default here because it reads naturally behind a centred subject; the edge painter continues the scene where the top and bottom are plain.
Questions
Does my photo get uploaded?
No. The picture is read, grown and encoded by code running in this browser tab. It is never sent to a server — there is no server to send it to. The only download that ever happens is the optional edge-painter model, which travels one way, from our model file server to you.
Will my subject stay centred?
Yes by default. You can also anchor the photo to the top or bottom in More options, handy when the sky or the ground should grow more than the other side.
Is a blurred top and bottom the only option?
No — continue-edge and mirror are there too, and the edge painter can continue a plain sky or floor. Blur is simply the safest default behind a person.