As I’ve seen some people hitting this site with the search term above:
- Keep looking in the direction of travel.
- A wall will come in from your right, with a painted bit you can target.
- stick the portal the travel beam comes out of onto it. wait for it to stop moving before you do this.
- You’ll fall into the beam, avoiding the mashy spiky plate.
Someone else’s video. The wall is the one that the crosshairs are looking at, /right/ at the end.