Documentation for these switches is here:
https://winscp.net/eng/docs/scriptcommand_put
https://winscp.net/eng/docs/scriptcommand_get

It took me quite awhile to find this buried in the History link (Black navigation bar on the website, last link on right). I may have missed it, but I don't see this information anywhere else in the documentation. It looks like you addressed the issue, and never got a chance to sync up the documentation. Please let me know if I am mistaken about this, but I tried it and it worked!

For those who are reading this and having the same issue, this was a feature added in version 4.2.2 beta back on 2009-06-28.

The release notes read...

New switches -append and -resume to scripting commands put and get to append file/resume transfer respectively.

The link to the document is below:
https://winscp.net/eng/docs/history?a=4.2.2

Hi,

I know that WinSCP will automatically resume a transfer if network connectivity is lost. I've tested that and can see it work.

But what about if the computer the job is running on needs to be restarted? Is there a scripting parameter to tell WinSCP to detect a partial file transfer and to attempt to resume/restart it? The reason I'm asking is we have a large file we need to transfer daily (via FTP), the file is 12GB in size and takes about 10 hours to transfer. It's possible that the server I'm running the transfer on could have a problem and need to be restarted.

In my Preferences setting, I already have resume/transfer to temporary filename set to Files above 100 KiB, and both Automatically reconnect options are enabled. But if I start a transfer from the command line, then kill the transfer by doing a Ctrl+C in the console window, and restart it, it starts from the beginning all over again.

Thanks in advance.