I'm not sure what you are asking with this question, but I will try to answer it. When Dana's life is in danger, she is sent back to the present time of 1976. When Rufus is in danger, Dana is then sent back to the past. She must protect Rufus since he is the future father of her first woman who is listed in her family records.
The first time Dana is called back by Rufus is when he almost drowns in the river. Dana saves him, but a gun is aimed at her head, so she's transported back to 1976. The next time she goes back, Dana is caught teaching black children to read, and she is beaten for it, which causes her to go back to the present. Another time, Rufus sold a slave he felt was too friendly with Dana, and Rufus punches her in the face when she protests. Dana feels cutting her wrists will be the only way to take her back to the present, and she's correct. The last time Dana is sent back, Rufus tries to rape her, and she stabs him. Rufus won't let go of her arm, however, and when she returns to 1976, her arm is crushed.
I think it's helpful to look at this question from the author's chair. Octavia Butler had to pull Dana out of the past and re-insert her at different times in Rufus's life. Instead of making this happen randomly, she chose to do it when Dana feels her life is in danger. That allows her to have Dana beaten and whipped later on without vanishing, as Dana learns what slaves endured and that they survived. It also allows her to put Dana in control when she slashes her wrists. I don't think there is any bigger meaning to it all -- mostly a mechanism to move the plot forward.