How Unyielding Will works(At school atm so can't watch the video) is at 75% hull you get a boost to antimatter regen/ability cooldown(I believe 25% or so) at 50% hull that becomes 50% to both(Taking 25% as first step) and at 25% hull 75%(Again taking 25% as first step) at 0% hull your titan becomes invulnerable and gets a massive boost to antimatter regen/ability cooldown for IIRC 4 mins(lvl 1 of the lvl 6 ability) after that it dies in a horrible fire and you are finally able to start rebuilding it 
Edit: While it is invulnerable it will still get xp, The only time I lost my AR titan on lvl 6 against vicious AI they just kept throwing units at it and it ended up being lvl 9 when the invul was finally over 
Was really fun just seeing my titan spamming his abilities like crazy Killed their titan in the process
