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Teleporting is a trick that makes the NPC to teleport straight into it's final walking destination.
There are essentially three modes for an NPC to be in. We'll call them Stand Mode, Follow Mode, and Path Mode. The latter is the one we're interested in. This is the mode where an NPC is headed for a specific point on the map, and will continue to try to get there no matter what block it's path. For example, even if you completely block an NPC's path while it is in path mode, it may try to strafe around the objects for a few seconds, but it will eventually teleport on the other side or straight to it's intended point, depending on how long it was delayed.
Taking advantage of this knowledge, it has been found that holding or lowering an object over a "sweet" spot just above and in front of the NPC's head while they are in path mode, most easily when they have just begun moving, will trick the NPC into thinking there is no way around the object and instantly teleports the NPC to it's destination.
Simply tap a NPC while it's walking with any object's flat surface (i.e. a book, plank). In OrangeBox it seems to be harder to teleport NPCs, but it's possible.
To teleport an NPC, you need to stand over him with the whole body. In HL: S, the flat object is Gordon Freeman himself. Just stand over the NPC and it teleports.