Well I'm still new to the mechanics of it also, so the best thing I could probably do is post the aside from the tutorial:

A little explanation: Forward Kinematics, or FK, is the way of moving bones that we first used. That is, rotate the upper arm, and its children (and children's children&#33 follow along. The opposite of FK is Inverse Kinematics (IK), where we move a child and the parents follow along. In reality, there is some fancy math going on in the background that tries to point the chain of bones toward the target. What's the target? For Auto-IK, it's whatever bone you have selected. In this case, the target is the finger2.l bone. What's the chain? It's the lineage of bones going all the way back to the great-great-great-(etc)-grandparent.

In our armature, when we moved the finger bone, all the bones in the chain tried to point to wherever we moved it. An orange line showed up, connecting the finger2.l bone to the spine1 bone. The orange line points to the root of the chain: spine1 is the highest parent of finger2.l, and the chain is everything between spine1 and finger2.l.

It would be nice if the spine didn't move so much when we moved the arm. We'll fix this by essentially breaking the IK chain at the shoulder so only the arm moves and the spine stays still.
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