How to use this GM correctly

Im not really sure how these two are supposed to be set up. How would i go about if
I want my animation to be double the speed when running?

If you’re making a gamemode and want to override the default behavior, use:

[lua]function GM:CalcMainActivity(ply, vel)
– your code here
end[/lua]

Otherwise:
[lua]hook.Add(“CalcMainActivity”, “the purpose of this code”, function(ply, vel)
– your code here
end)[/lua]

Hopefully that answers your question…