Request Manager is another new feature in SharePoint 2013 .
Request Management is used as a Service instance although there is no associated Service application. All configurations are performed using Windows PowerShell commands. Request Management is scoped and configured on per web application basis.
What it does?
*Deny potentially harmful requests from entering a SharePoint farm.
*Route good requests to an available server.
*Manually optimize performance.
To enable Throttling in Request Manager.
Get-SPWebApplication | Get-SPRequestManagementSettings | Set-SPRequestManagementSettings –RoutingEnabled $true –ThrottlingEnabled $false –RoutingWeightScheme Static