answer pack 2: I will just repeat some points from the presentation itself - again, it's in the link in video description on YouTube.

I love mocap, and I love to base things on mocap. however, pure mocap limits us. what if I want more than one rig? mocap on each new rig, or retarget to each new rig, ok. what if I want to crouch? mocap crouch, ok. what if I want to limp? mocap limp, ok. what is i want to crouch and limp? while carrying weapon? on a different rig? I can make a bunch of mocaps or retargets. or I can create an IK Rig rule to turn male into female, another to turn walk into crouch, another to carry weapon, another to limp. then apply them all together to get male walk into female crouch with weapon, limping.

another example: each character gets his own rig, yes; but also gets his/her own muscle strength. depending on this, the way he walks, the way he carries weapons etc, changes. so as your character gets tired, the way he carries his sword changes. if every sword has different length and different weight, they will all be carried differently based on sword, character, and your stats. more than that: every sword will be swung differently. throw in the idea that you don't fence on plain surface (like your mocap actor); you swing uphill and downhill, and your opponent's height may differ from yours. you should not be playing the same animation.

the motionbuilder does retargeting, as many other software. the issue is that such retargeting is baked into your animation. what IK Rig tech is made for is to retarget at runtime, getting input on how to change the pose based on animator's wish, but also based on current game conditions (environment, ground, character stats, etc); and do this for every different type of character. so every character can have its own rig; yet all animations will still work for all rigs.