[As additional resources, links to book reviews and book purchasing information can be found beneath the quotations when this information is available.]
"The shoulder-in is used to supple and encourages use of the horse's hindquarters."
"Riders most frequently make the mistake of leaning towards the inside. Amongst other inconveniences, this loads the legs which are under the greatest strain. This may be avoided by leaning on the outside stirrup."
"An incorrect shoulder-in, or shoulder-in exercise asked for too early, doesn't lead to passive flexing of the haunches."
"If the rider tips away from the working hind leg and sits to the outside, the purpose of the shoulder-in is lost."
"From my point of view, a correct shoulder-in comes seamlessly from correct leg-yield through shoulder-fore as training progresses."