The purpose of foreshadowing can be explained two ways, and this is important because the viewpoint taken affects a reader's expectation of being able to predict events and outcomes from foreshadowing. For the student of literature, learning to identify foreshadowing and attempt predictions is important to success with literary analysis. For the creative writer, implanting foreshadowing that is so subtle that readers are still surprised by the surprise ending or by the plot twists or by the unexpected happy ending is important to the success of the crafted work. The truth is that very often foreshadowing completely escapes us as readers, even seasoned readers, because it is successfully crafted into the fabric of the story. ("The Lottery" by Shirley Jackson is a great example of a story with subtle foreshadowing that escapes us as readers so that we are shocked by the surprise ending.)
An example of such subtle foreshadowing from "The Leap" is when the narrator parallels the late Mr. Avalon with her father, the "specialist in arms and legs," as a way of foreshadowing one early event and one later one. First, she foreshadows Anna's marriage to the doctor through the parallelism of her being "trained to perform" by the Avalons and of the doctor's insisting "on teaching her" to read and write, giving her "one form of flying for another." Next, with the same parallel, the narrator foreshadows the mode of rescue that is going to be needed later in the story when the "upstairs room [is] cut off by flames" and the "one staircase" is "lost." The foreshadowing for the later event lies in the doctor's specialization "in arms and legs" as Anna bares her arms and legs, climbs with them along a tree branch, springs with them to the roof, dangles by them from the gutter in order to grab her daughter (the narrator), holding her safe in her arms and legs for their joint leap, falling (with time to think) to the painted "circle" of the "fire fighter's net."
[S]he made her way up and ... inched the length of a bough .... She was hanging by the backs of her heels ... [then] we flew out the window, toward earth, me in her lap, .... Then I wrapped my hands around my mother's ... [and] heard the beat of her heart in my ears, loud as thunder,....
This foreshadowing is so subtle and so much a part of the story that it's doubtful any of us catch it the first time, so it is doubtful that it allows us to predict anything about upcoming events or the outcome of the story (it may be recognized only as recurring motifs of fire and of arms and legs). Yet, the foreshadowing is there so that when we go back and reread, we can say, "Ah. There it is. She didn't trick me with an unsupported twist and surprise ending." It's that feeling of being tricked that readers and creative writers both want to avoid.
Two other foreshadowings are when the narrator foreshadows (1) that there will be a fire in the narrator's childhood room (now sewing room) and (2) that her mother will be her rescuer using "double somersaults and heart-stopping catches." (Anna's heroic rescue is an important element of the plot; don't be surprised if there are more than one instances of foreshadowing setting it up: e.g., "catlike precision of her movements.") In a very deliberate (opposite of subtle) foreshadowing, the narrator points to the fire that will be in her room: "I hear the crackle, catch a whiff of smoke from the stove downstairs and suddenly the room goes dark, the stitches burn beneath my fingers."
In this loving tribute to the inner qualities and physical feats of the narrator's rescuer, Anna, the fire and Anna's subsequent rescue are the central events of the plot; these are what all other events and descriptions lead up to, so they are significantly foreshadowed. That the narrator will be rescued by her own mother, who will hearken back to and draw upon talent, training and skills used for the last time when her beloved first husband died, is the ultimate point of this tribute, so it is foreshadowed in a number of ways: e.g., if the narrator didn't know the truth, she would "tend to think that all memory of double somersaults and heart-stopping catches had left [Anna's] arms and legs."
As to whether these and the other instances of foreshadowing (e.g., "a month and a half before her baby was born without life," foreshadowing another marriage, husband and child) help as clues upon which to base predictions of outcomes, I confess I originally read the story three times before I found enough foreshadowing to stop feeling utterly tricked and puzzled by how things managed to develop as they did. The fact that I did eventually find enough foreshadowing to finally understand the structure of the story attests to the embedded nature of the foreshadowing—embedded in the fabric of the story—and predicts a well-crafted story and a happy creative writer: Louise Erdrich.