The midpoint exists to solve a specific problem: the middle of a story is the part most likely to become a sequence of obstacles with no shape. Putting a genuine reversal at the centre splits act two into two halves that behave differently — reactive before, active after — which gives the middle a direction rather than a duration.
It usually takes one of two forms. A false victory, where the protagonist appears to get what they wanted and it turns out to cost more than it is worth. Or a false defeat, where they lose and the loss reveals what the real problem was. Both convert a want into a need, which is why the midpoint is where character work usually happens.
In a short piece the midpoint is often the only turn there is room for, which makes it the most valuable thirty seconds to get right.
What happens at the midpoint of a story?
The protagonist gains information or experiences a reversal that changes them from reactive to active. Often a false victory or a false defeat that reframes what the real problem is.
Why does the middle of a story sag?
Almost always because there is no midpoint turn, so the second act is a list of obstacles rather than two halves that behave differently. Adding a real reversal at the centre fixes more sagging middles than any amount of cutting.