Johnnie, Lina and Beaky are playing anagrams. Lina spells out the word MURDER with an M as the first letter. The shot cuts away and then back again, and now the first letter is an inverted W.
Joan Fontaine won a Best Actress Academy Award for her role as Lina McLaidlaw Asygarth in this film. This is the only Oscar-winning performance in an Alfred Hitchcock film.