A magazine article listing "50 sex tricks" is expected to present little known techniques for making sex more enjoyable. The second panel sets up the expectation that trick number one involves some kind erotic use of the hands. Panel three reveals that it is merely a cheap parlor trick implausibly adapted to the genitals. That is the joke.
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Ms.ShowMe: What're you reading? 50 sex tricks?! Hah, show me some!
Ms.Read: Ah.. it's not really that great of an article
Ms.ShowMe: Come on! Just show me the first one
Ms.Read: *Sigh* ok... You know the ol' disconnecting thumb trick?
Ms.Read: Like that, only with a penis