Hi Bruno,
this is my official understanding on it, and unless others on the rules committee state otherwise and we come to a different interpretation as a group (I doubt it), it’s official.
When it’s before the throwing motion, the thrower can decide (in most cases) to let play continue by calling contact, or to stop play by calling foul.
On the hand block:
If the disc is still in the thrower’s hand, it is a foul, even if you get your hand in front of the throwing motion and stop it a millisecond before the contact. The only time this is an offensive foul is when your hand was at the place of contact way before the contact, usually at the time the forward movement of the disc in hand starts (this is when the throwing motion starts, the pivot is not part of the throwing motion). This used to be a matter of much discussion, and both USAU and WFDF have now clarified that this is almost always a defensive foul. A “proper” hand block is a block after the disc is released (i.e., after the disc has left the hand so far that the thrower has given up control to the point that he could not hold on to the disc anymore, usually when the disc is coming out of the grip by itself), and in this case it does not matter if the defender is moving etc. . The video that you are linking appears to show such a proper block, and I am sure this is the only thing they were discussing (“the disc was still in my hand…” “are you sure? I don’t think so…”).