Thanks you for your answer.
I tried Additive and Override, the problem stay the same, it does not give me any transitions between animations because of I cannot make a transition link from my locomotion blendtree to my Hand throw animation, the layer only give me the choice to do transition between an Empty state (with no animations in it) and the Knife throw animation.
I also cannot use the video you have linked because of it modify the animation itself and my problem is only the transition between 2 animations that are in different layers.
Pardon my lack of knowledge, Im totally lost actually !
Ps : The video finaly gave me and idea, I will try that thing : When I press the throw button Ill set up a IK goal right before starting the throw animation, I'll see how it looks.