When she wants to tease her slave, she makes videos for him to take on his business trips. She records herself playing in long strands of spit, knowing that it turns him on. When he leaves her presence, she forbids him any pleasures until he comes back. So as he watches her play in her spit, his cock is hard and he can't have any relief until he comes back to her.
When she looks for a slave, she knows what he wants. She knows what will drive him insane with the desire to bow to her wishes. She knows that he wants her warm, slimy spit. She is happy to sit in front of a potential slave and spit into a jar. If he doesn't want to play in her spit, she knows that he is not the slave she needs. When she finds him, he will give her anything to get that warm jar of spit.