It's just like hooking a TraceLine. This is how a TraceLine usually works in Half-Life.
Code:
// PRE HOOK
UTIL_TraceLine(vecSrc, vecEnd, dont_ignore_monsters, ENT(pev), &tr);
// POST HOOK
if (tr.flFraction != 1.0) ...
If you hook it as a pre hook, then any changes you make to the trace are just going to get overwritten as soon as it finishes the trace. It might actually change the results of the last trace sent before this one.
But if you hook it as a post hook, then you can change the results of the completed trace before the game gets a chance to look at them. So in this example, you could change flFraction in a post hook and it would effect the if statement.
AddToFullPack works the same way, ish.
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