I think you might be confused. 'B' is a *Pine* command to resend a message,
leaving the original To, From, etc. headers intact and adding a Resent-From,
Resent-To, Resent-Date, and Resent-Message-ID to reflect the new source and
destination of the message.
While this is a great feature, I don't think it is a Yarn feature.
In yarn, 'B' extracts a uuencoded file from the message.
You could use 'm' to forward a message, but it mucks up the headers.