Slide film cross processing is a technique that involves developing slide film (also known as reversal film) using chemicals intended for use with colour negative film. This process results in a number of unique and interesting effects that can be quite different from those achieved through traditional slide film development. 

One of the most notable effects of slide film cross processing is a shift in colour. Because slide film and color negative film are designed to be developed using different chemicals, cross processing can result in a reversal of colours, with reds appearing as greens and blues appearing as yellows, for example. Additionally, because slide film is designed to be projected, cross processing can result in increased contrast and saturation, giving images a more dynamic and vibrant look. 

Another effect that can be achieved through slide film cross processing is an increase in graininess. Because slide film is designed to be used in a different manner than colour negative film, it can have a different grain structure, which can be accentuated when cross processed. This can give images a more gritty, textured look. 

There are a number of reasons why people might choose to cross process slide film. For some, it is simply a creative choice, as the unique effects achieved through cross processing can add an interesting dimension to their images. For others, it is a way to achieve a specific look that can't be achieved through traditional film development. Additionally, slide film is becoming increasingly difficult to find, cross processing allows photographers to continue using the film they have, even if it is not the intended film for their camera. 

Overall, slide film cross processing is a technique that can produce a wide range of unique and interesting effects. Whether you're a professional photographer looking to add a new dimension to your work, or an amateur looking to experiment with different film stocks, cross processing is definitely worth considering.