This is the name of a sexual act: Towards the end of doggy-style sexual intercourse, the male proceeds to ejaculate on the head of the female. After this is complete, he then proceeds to drop a weight on the head of the female, thus completing the "Harriet Tubman." This is derived from the fact that Harriet Tubman had something dropped on her head as a child, explaining her glass eye.
Dude 1: Damn dude he dropped a weight on that bitch after underground railing her.
Dude 2: Dude he totally pulled a "Harriet Tubman."