Pre flashing paper is a useful technique which I use a lot, but isn't really a contrast increasing measure. It's more a way of conning the paper into increasing its dynamic range. The basic idea is that the paper needs a certain amount of light to respond at all and some highlight areas might not quite get it in the enlarger exposure, leaving them blank white in the final image. So you give the paper a brief flash of white light before the proper exposure, of a level which is not enough to cause it to respond, but only just, so that any highlight detail will then push it over the edge. It is particularly useful in areas where the detail is too complex for burning in and the effect on shadows is virtually nil. I've never tried it, but it might well be useful with paper negs, which can give a rather chalky look, and you can pre flash several sheets at once. Be aware, though, that the effect fades if a significant amount of time elapses between the pre flash and the actual exposure so you would need to use the prepared paper within a day or so of the pre flash.