the break in period is that in the first 100 hours or more.. +/-
the phosphors in the set burn hotter and are more prone to real burn in and serious IR.
so the burn in period a.k.a break in period is to age the phosphors evenly across the board as to have it so your set is even. so when the period is over the uniformity across phosphors are good and you are less prone to serious burn in and or IR.
this is actually pretty recognized as the truth.
even manufacturers circa 2007 up to say this.. but i think it's best anyways.. just to watch full screen material a.k.a 1:78:1 in the first 100 hours.