==THIS IS A THEORY==
Click to see
Here's your problem, the snapback is probably because the bone tree isn't correct, the first 2 bones need to be at (0,0,0) which, in lucas' case is the TopN and the EyeYellowM (Usually called BodyM). The TransN's index must be one higher than the BodyM (Or EyeYellowM) and be at (0,0,0) and can go to (0,0,*However far the character moves*) during the animations. I noticed in the video that the TransN wasn't set to (0,0,0 for its Tpose, which may be what's causing the problem.
Also, the ThrownN bone stays with the character, while the first 2 bones stay at the snap-back point.
You found most of this out in the video towards the end, I started writing this as soon as I saw that part of it in the vid xD.
Click to see
Here's your problem, the snapback is probably because the bone tree isn't correct, the first 2 bones need to be at (0,0,0) which, in lucas' case is the TopN and the EyeYellowM (Usually called BodyM). The TransN's index must be one higher than the BodyM (Or EyeYellowM) and be at (0,0,0) and can go to (0,0,*However far the character moves*) during the animations. I noticed in the video that the TransN wasn't set to (0,0,0 for its Tpose, which may be what's causing the problem.
Also, the ThrownN bone stays with the character, while the first 2 bones stay at the snap-back point.
You found most of this out in the video towards the end, I started writing this as soon as I saw that part of it in the vid xD.