Microwaves only penetrate to a depth of around three or four centimetres, so anything deeper is heated indirectly, by conduction from the outer layers of food. The cooking instructions on a ready meal might say to heat it on full power for five minutes and then leave to stand for another two.
The first phase adds enough energy to cook your lasagne, but when it comes out of the microwave, that energy is unevenly distributed. Leaving it to stand allows the heat to make it to the centre, where it will kill off any bacteria. So always respect the instructions.
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