I wrote a blog post about the technical details of movement on downward slopes:

Source vs GoldSrc Movement: Downward Slopes

Here’s a snippet:

The slope-related movement physics in the GoldSrc (Half-Life 1) engine and the Source (Half-Life 2) engine are fairly similar. However, there is one caveat around moving down slopes: in the GoldSrc engine you will bounce down slopes somewhat often, while in the Source engine you will smoothly move down almost any slope unless you collide with them while moving extremely quickly.