I was asked about the rise on a shetland fleece after I used the term to describe what I sort for when deciding if a fleece is show/sale worthy. Some breeds will shed their fleece every year, and the rise is the weak spot in the fleece that allows it to happen.
Some people will harvest the fleece by plucking it off, or "rooing" after the sheep has hit the rise. You can tell the sheep has hit the rise when you grab her by her wool and a clump of it comes off in your hands.
Rich and I tried rooing once and my hand was aching by the time we finished so we just take the chance that most of the sheep are in the midst of the rise at shearing time. Here is one of many very cool photos from the Shetland Museum of people rooing their shetlands:
The problem is if you shear too soon before you hit the rise will result in a difficult time shearing and too late results in a weak spot at the base of the staple that has the same effect as second cuts.
Kelly Bartels at OK Acres did a really good post on this with current photos of a sheep she rooed.