Separate Shell Session
Some features allowed with SCP protocol are not supported by SFTP protocol. This includes execution of remote command and direct duplication of remote file. To support these features with SFTP protocol, WinSCP can open a separate shell session, to perform the commands on. The shell session is opened automatically (with suppressible confirmation) when you attempt to use any of the mentioned features. After the command is executed, the shell session will be kept in background for future use.
For the feature to work, WinSCP must be able to open a shell session similar to one used by SCP sessions. So while you are using SFTP protocol, you may still need to alter options on the SCP/Shell tab of the Login dialog if the defaults do not work. Also, for the feature to work, the shell must use the same path syntax as the SFTP server.