I've copied the seventh page from the rowing technique download available on you myCRU team page below. Please read it carefully and then watch the videos.
Blade work is a skill which has direct impact on the movement and speed of the boat. For this reason, many coaches pay much more attention to correct blade work than to the body motion. Yet, the blade work is a direct reflection of what is happening inside the boat. It is possible to change either body motion or blade work and see improvements in the other element. An example of good blade work is shown below.
During the recovery, the blades travel toward the bow in a smooth, horizontal plane at a steady height. The rower should allow enough clearance for easy squaring of the blades before entry without skimming the surface of the water.
The turning of the oars so that the blades are perpendicular to the surface of the water is called "squaring." Ideally, squaring should start as the hands go over the ankles. Squaring should be executed gradually at a constant speed, during the last part of the recovery. It should neither slow down or stop the motion of the blades ("hanging") before entry.
The entry into the water should be quick, as a continuation of the recovery. It should be well synchronized with the speed of the boat, without too much back splash or front splash. For beginners, however, some minimal back splash is suggested, to make certain that they don't miss the water. The idea is to use the gravity or weight of the oars, instead of power, to place the blade into the water. "Scooping," the entry of the blades into the water, should be followed with immediate horizontal power.
The "early roll up" as referenced in the title of this post is in contrast to a "snap" roll that many beginners use. Below I've illustrated what I'm calling a "snap" roll. The rower leaves the blade feathered until the last possible moment and then tries to immediately square it, sometimes even while the blade is entering into the water. The problem is that it's hard to feel when the collar falls on the flats of the oarlock when snap rolling the blade. If you rotate not enough or too much problems result.
The early roll up allows the rower to feel the collar drop onto the flat section of the oarlock and ensures when the blade enters the water it will be perfectly square. Below is a view from the port side of reasonable blade work in a pair.
Here is a slo-mo video the US Men's eight practicing before the world championship, followed by a close up of the blade work. Again, note there is not "snap" to the squaring, but rather a smooth easy and early roll up.
On the slow motion video also notice the body preparation at the start of the recovery and the relative handle heights between the recovery and the drive. The handles move horizontally with no "arching."