How does Quake 3 handle collisions when the actor collision area changes?

[Asfand Yar Qazi]

Hi,

I'm learning game engine design by writing a simple game engine in
Ruby, and wanted to ask something about Quake 3, so thought I might as
well ask it here (sorry if it's inappropriate).

How does Quake 3 handle collisions, when the model (and hence
collision area) changes frames, or directions?  E.g. a polygon is
right up against a wall, and it's not colliding with it.  The actor
changes direction, e.g. by turning away from the wall.  We assume this
is the player's character.

Now, in the next 'OnTimer' event thingy, a part of the Actor's model
ends up actually inside the wall it was standing  right next to.  What
does the Quake 3 engine do here?  E.g. does it 'pause' the input
handling of the actor while it smoothly moves the actor away from the
wall, before reenabling input handling again?  Or something else?

What are the main files I should look at to see the Actor procesing
logic the Quake 3 engine employs?

Thanks for the info, sorry if I sounded a bit confused,
        Asfand Yar