If needed, you can log in to the Reseller Control Center or Control Panel on behalf of your customers directly from the Provider Control Center without the need to open your browser and enter login /password.
The most general way to log in to any of resellers/customers tools, is opening the customer or reseller account details (in the Account Director - Customer Manager or Reseller Manager) and click the Login to CP or Login to RCC button respectively.
The logging button is also available both at Container - General Settings and Domain properties - General Settings screens.