One of the oddities of ICANN's Rules for Uniform Domain Name Dispute Resolution is that they require the complainant to affirm, in the complaint, that a copy of the complaint has been sent to the domain name holder (Rule 3(b)(xii)).
In theory, this requirement could give a fast-moving domain name holder the chance to transfer the domain to another person before the proceedings formally begin (from which point such “cyberflying” transfers are forbidden under Rule 8(a)).
WIPO panelists (at least) have developed a jurisprudential solution to this problem, illustrated by the majority decision in Gloria-Werke H. Schulte-Frankenfeld GmbH & Co v. Internet Development Corporation and Gloria MacKenzie.
In that case, the panel treated the domain transferor as the “primary” respondent, whilst acknowledging that the transferee also had an interest in the domain name.
As the UDRP system has no formal system of precedent, you should be wary of relying upon this non-Rules based argument. The best way to reduce the risk of having to do so is to notify the relevant arbitration centre and registrar electronically, but put the registrant's notice in the post.