jk_cp -j /var/www/clients/client2/web1 /bin/bash
to test it. See here for details:
you can configure the applications that get copied to the jail in the jailkit settings in ispconfig.
Btw, I wont use sftp as it requires you to create a shell user. If you want to get secure file transfers, use ftps which is ftp over ssl, this is as secure as sftp but does not require a separate shell user and jail and works out of the box with a ftp user account.