To be honest this documentation is the part I'm most unsure about, since the Y.io documentation on the "xdr" config param is in a state of flux. The current Y.io docs imply that the option is deprecated: http://imgur.com/l1zEIZQ I know that SWF's are gone from YUI (precluding {use:'flash'}), but I believe native XDR (i.e. using XDomainRequest in IE 8/9) will remain supported?

In either case I believe just positioning this as "pass through to Y.io" option will be clear enough to users who need this functionality.