I haven't figured out how to warp to exact positions yet. But it's probably dependent on the position you're standing in before the execution animation, and the direction you're moving & facing during/after the animation. These things apply to the metro warp too, but in that case it's with the dialog instead of an animation.

How to do biker warp:

1. Get biker to throw his second knife in the upper half of the golden nipple on the right wall, you want him to be as close as possible to the top wall while he's pulling his knife out of the right wall.

2. Hit him from the bottom (biker's right side) and he will get stuck in the top wall and crawl in a circle forever.

3. Execute him when his legs are facing diagonally to the top-right through the wall, you will clip into the wall's collision box during the execution animation and get warped.

Done!

After messing around with this again, I'm pretty sure this is how it happens:

• Some of the trash bags have wonky collisions, other trash bags won't work.
• Stand in a good position within the wonky collision box of the trash bags before the dialog.
• The direction you move and the direction you're facing has an effect on it too.
• After closing the dialog, the game tries to find the position of the player.
• If you are inside those wonky collisions the game gets confused and warps you.

Unfortunately, it still seems pretty random and hard to pull off :(

I've noticed another thing. Those trash bags can change appearance. In all the cases of this warp working, you can notice the lower trash bags are colourful. This could make the collisions more wonky or something?

I messed around with this and found out that it may be triggered somehow by the actions you make during/after exiting the dialog. The collisions around the trash bags to the left of the dumpster are a little wonky, maybe that has something to do with it too? You may have to stand in the correct spot for it to work.

Currently I've found the most consistent method is holding W + A (walk up and to the left) while moving the mouse up and to the left and mashing left click.

Here's a video of me performing the warp a bunch of times.

Hope this helps somehow.