Effect of Leakage inductance in flyback converter?
In an ideal world, we would be able to make transformers where all the magnetic energy in the primary winding can be liberated from the secondary winding. This is nearly true - probably about 95% true - but that small percentage of energy that isn't passed through to the secondary winding (on a flyback design) produces a high voltage spike when the primary winding is open circuited.

Flyback designs "impart" the primary with energy in the 1st cycle of operation by connecting the primary directly across the incoming power supply. Once a certain current is reached (or a certain time span has elapsed), the energy in the primary is \$\frac{L i^2}{2}\$. Where L is primary inductance and i is the current flowing in the primary.

The 2nd cycle of the flyback convertor starts with the primary being open-circuited - most of that energy is transferred to the secondary and this forward biases a diode that transfers the magnetic energy into a capacitor. The capacitor is basically where the output is - it connects to the load.

If the flyback transfers 100,000 times per second and its primary energy is 20uJ, then the flyback is transferring a power, to the load of 2W (frequency x energy). But not quite, maybe 5% of that energy is not liberated by the secondary and this, when the primary circuit is open-circuited (usually by a MOSFET) will create a significant back-emf that can, in some circumstances destroy the MOSFET.

So this is what leakage inductance can do and on high-power designs it can be a significant problem.