In simple terms, mast inversion is when the mast bends forward instead of aft. When the mast is inverted, you no longer have the expected aft curve that matches the luff curve on your sail. When your mast is inverted, your sail shape is "not fast". This is normally, the worst you see.

When your mast is inverted, you can also get "S" shapes and even more complicated shapes. When the mast starts to move or oscillate in "S" shapes, that is very bad, your mast may fail.

Raking the spreaders aft, with tight diamond wires, puts a force on the mast that try's to bend it the way you want it to and (I am told) stiffens it against motion in other directions.

The problem is worse on the 5.5 uni or NACRA 18sq, which use the same extrusion but, the mast is longer and square top mains make it worse. I have seen 18 sq's rip the top of the sail out or break the mast. This normally happened, downwind in heavy puffy air.

Minor inversions happen on a lot boats and are not that bad. Big inversion or complex shapes that move are very bad. When the mast is oscillating, the wrong load in the wrong direction, cab break the mast.

Tapered, pre-bent masts are the next step. You make the mast easy to bend in the directions you want. at the same time you make it stronger down low and stiffer in directions you don't want it to bend.