Marley asked me to elaborate on this a bit. It's a bit hard to explain, so I might try to get some video of it in the near future. For now, I'll give it a shot! :)
Finger tugging (as I call it) is literally the dog tugging on my finger! I curl my index (usually) or middle finger into a C shape and loop it behind the dog's 4 canines. This way, my finger is in a "no crush" zone, while the dog still has a good grip to tug. This is also really handy since I can just slip my finger out if things get too riled up (so it's a good tug game for dogs without a good "out").
This is really nice since it doesn't require anything I don't already have with me (dogs' teeth, my fingers) on a regular basis. It's NOT for dogs who don't know how to be reasonably gentle with their mouths!!
This video shows how to clicker train the dog to take your finger in his mouth. This is about where my finger sits, only I curve it so the dog has some tug grip. You can see at 2:30 when the dog is holding, that's where I start tugging gently.
Please do always feel free to ask about something if it's unclear! I don't always know what is and isn't obvious. This blog is mostly a catalogue of West's adventures, but it should be educational too!