WARNING: Do not skip the Doppelganger cutscene! Although, sometimes you can. Update: As long as you don't hold onto the target and shoot buttons, you will be fine...
This glitch in Tomb Raider Underworld is supposed to work in all versions, regardless. How it's done is simple: I did something called a "Delayed swan-dive". It allows you to swan dive near to ledges without moving and then roll in mid air. But while this is happening, you can also JUMP in any direction!
1. Just do a "Delayed Swandive" near the edge and jump to the left. For Xbox 360... Press A to jump and then press B to crouch at the VERY LAST moment. Timing has to be right.
2. Climb up the ledges near the chimney.
3. You will get a checkpoint that you weren't actually supposed to get.
4. Go make your way over to the tech room.
5. Drop down and VOILA!
6. The cutscenes can be skipped, as long as you don't try to hold down onto the target and shoot buttons while trying to do so.
INFO: The Delayed Swandive is a special swan dive - It allows you to do a normal swan dive, but without Lara moving from her position. You will then walk in mid-air for a sec, and then still have time to jump in any direction.
* Nintendo Wii Thailand bug users need not the patch anymore! :)