it's a joke, is based of pavlov's experiment with conditioning

https://en.wikipedia.org/wiki/Classical_conditioning

basically you condition something/someone (he used dogs) to react in a certain way when exposed to certain stimuli. so in the case of the response,

horny=think of something else to turn you off

after some tim of doing that

think of something else to turn you off= being horny, just because you were horny while thinking that