Leakage inductance is caused by that part of the primary flux that doesn't make it to the secondary. That, and the stray capacitance across the primary, limits the high-frequency performance of the transformer. Using a clever combination of split and overlapping windings, this can be optimized, but what is the best combination for a particular transformer? Pierre Touzelet has analyzed the situation. This article is heavy on math but the conclusions point to a set of rules that are surprisingly simple.