In Chapter 10, Douglass describes how enslavers purposely make holidays unpleasant so that enslaved people will not want them. He calls the institution of slavery "fraud," which 19th-century white readers would have taken as a metaphor:
I have said that this mode of treatment is a part of the whole system of fraud and inhumanity of slavery. It is so. The mode here adopted to disgust the slave with freedom, by allowing him to see only the abuse of it, is carried out in other things. For instance, a slave loves molasses; he steals some. His master, in many cases, goes off to town, and buys a large quantity; he returns, takes his whip, and commands the slave to eat the molasses, until the poor fellow is made sick at the very mention of it.
Douglass is describing what amounts to a real theft. The enslaver, in this instance, is stealing the simple human joy the enslaved person feels when eating molasses. Douglass is making the point that not only does the institution of slavery defraud enslaved people of the monetary compensation they ought to receive for their labor, but it also defrauds them of human experiences that ought to be universally available (basic things like the pleasure of eating something sweet).
In the 19th century, white people did not always see enslavement as a real form of theft. Capitalism was booming. It was often used as a justification for preserving the institution of slavery: apologists for slavery argued that abolition would constitute over-regulation of the market. Discussions about labor rights were emerging and making for thornier debates about the ethics of enslaving people, but enslavers were not necessarily considered by their neighbors to be literal fraudsters. Still people were highly suspicious of more easily recognizable forms of fraud, which were pretty easy in an unregulated market. White readers would have been alarmed at the idea of unchecked fraud because it threatened to throw the entire economy out of balance. By comparing enslavement to the kind of fraud white readers were already afraid of, Douglass turned the whole system into a threat to them as well as to Black people.
In Chapter 11, Douglass critiques the way abolitionists operating the underground railroad proclaim themselves publicly. Douglass uses an implicit metaphor to claim that a more useful and indeed fairer system would be to keep everything as secret as possible:
I would keep the merciless slaveholder profoundly ignorant of the means of flight adopted by the slave. I would leave him to imagine himself surrounded by myriads of invisible tormentors, ever ready to snatch from his infernal grasp his trembling prey. Let him be left to feel his way in the dark; let darkness commensurate with his crime hover over him; and let him feel that at every step he takes, in pursuit of the flying bondman, he is running the frightful risk of having his hot brains dashed out by an invisible agency.
Everything Douglass imagines enslavers enduring under this more secret system is something enslaved people already experience, especially when they are using the underground railroad to escape. Enslavers try to prevent literacy, keeping enslaved people "profoundly ignorant" of the means of escape. Enslaved people must live in a state of hypervigilance, always looking out for "myriads of invisible tormentors" in the form of patrols who were payed to search for so-called "fugitive slaves" who had "stolen" themselves away from their enslavers. Escape often had to happen under cover of darkness, and it took people through unpleasant and even life-threatening terrain they had to feel their way through. Many, many people who ran away from enslavement were caught and killed or severely beaten.
By imagining the horror a more secret underground railroad would visit on enslavers, Douglass thus indirectly describes the horror enslaved people face on the underground railroad. He critiques white abolitionists operating the underground railroad for congratulating themselves too soon. If they were truly interested in justice, the metaphor suggests, they would work harder to help more enslaved people. Perhaps even more importantly, they would hold enslavers accountable for the atrocities they have committed. As things stand, enslavers do not have to endure the horror Douglass describes in this passage. They continue to operate as they please, only occasionally losing a little wealth in the form of someone who escapes enslavement. Meanwhile, that person who escaped must bear all the punishment the enslaver deserves to experience.