This trick is on all games on the HL1 engine, the trick can be used on the ground when running in NS for example... so it's not just ladders. It's called "wiggle walking", increases your speed. It was initially seen as an exploit, as was bunnyhopping.
You can still "wiggle walk" go at a 45 degree angle to the wall and it still works. you guys when you fixed ZP only made it so it was a 90 degree angle to the wall of course only I know this secret hehehehehehe