The croup is moderately sloping, the ideal being thirty (30) degrees from the horizontal. The croup angle is determined by the sacrum vertebrae and the first two tail vertebrae (so you must keep at least the first two vertebrae on the tail.

Croup angle determines the distance the rear assembly can reach backward and forward for maximum drive. A dog with a steep croup (45 degrees) will have less range of motion, but be stronger in the rear (good for climbing uphill) but a dog with a flat (20 degrees) croup will have a very wide range of motion, but no real power (good for distance travelling). The Aussie needs a mix of both. Tail set becomes important at this point.