Welcome to EF, jktz90. By is a preposition . All prepositions are followed by the gerund: You learn by do ing.
New words, one handy idiom, and a 2-minute quiz — delivered to your inbox to keep your streak alive.
jktz90So its not "forbidden" to write: "The author uses this kind of rhetorical device to provoke symphaty for the unequal situation of blacks during the last centuries by explaining [...]"No. What made you think it was?