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.
We do carry out checks before every event but random checks will also be carried out throughout the season and after races.
Gear Restrictions for all road events:
Under 14 riders (Youth B) will be restricted in accordance with British Cycling regulations which states a maximum of 6.45 m per pedal revolution when in the highest gear.
Under 16 riders (Youth A) will be restricted in accordance with British Cycling regulations which states a maximum of 6.93 m per pedal revolution when in the highest gear.
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
It's important to note , no local records can be claimed whilst riding de-restricted.