Preflashing paper for making in-camera paper negatives or paper positives is a way to lower contrast when using a high contrast media like black and white photo paper.
If unflashed paper is used (straight out of the box and into the camera) to make in-camera images, the photos will be very high contrast. There will be areas in the negative image that are blocks of pure white with insufficient exposure to define texture or shape. (in the case of a reversal for a positive, there would be blocks of pure black shadows with no detail)
Before being loaded into the camera to make the image, adding just enough light to the paper moves the whites to the brink of changing tone. The preflash exposure values are determined by making a test strip to find the values that moves the paper to a discernible tone off of white. In the case of reversal process paper positives, the value would be found where shadows were just lighter than black. I have found that these two values are not the same.
For example, I use an enlarger to preflash paper. I use a specific height for the enlarger head and an aperture of f8. For paper negatives (that will be scanned and reversed digitally) I use a 3 second preflash. As you can see from the example above, a 2 second preflash is still indistinguishable from white, while a 4 second preflash has some density already.
For paper positives using a reversal process, I use a 6 to 12 second preflash (depending on the contrast of the subjects I plan to shoot). As you can see from the example above, even with a 10 second preflash (the end of the test strip), the d-max of the blacks is still close to being reached.
For paper positives (using Ilford Harman Direct positive paper) I use a 2 second preflash.