Essentially it's manually adjusting and limiting the range of gears that a child has the use of.

British Cycling require this to be done for 3 main reasons.

1. It ensures all riders compete together on a fair and equal standing
2. It helps to reduce the risk of overuse injuries and avoid strength imbalances in young riders; it may also help riders to develop good pedalling technique
3. It encourages young riders to race using tactics as opposed to using bigger gears to go faster. This will help to support the riders in learning new techniques which they will need throughout their competitive career

Radom checks will also be carried out throughout the season and after races.

For more information, see The British Cycling 'guide for riders and parents'

For chainring sizes and combinations see this calculator

This video shows the process of how to restrict gears