This is what my book says, dunno if it helps:
How do I ask for lengthened strides?
The important issue with lengthened strides is that you"rally up" your horse and excite him, thus he will pick himself up and move forwards with the necessary enthusiasm and impulsion. Only you will know if it is the use of your seat, your legs or both that will achieve this. Remember, however, that "less" is always "more"!