You already know how this kind of curve works. Just like in Photoshop. Click on the curve to add a point. Move the point up, down, left, right, wherever you need to get the desired effect.
When you hover over a point the cursor will turn into a horizontal bar with up and down arrows indicating that you can now drag the point where you like. As you adjust the point the readout in the upper left of the curve display will show you the position of the point. It's important to note that Lightroom 3 Beta 2 is still using a 0% to 100% scale instead of the 0 to 255 scale for measuring tonal values.
If you hold down the Option key on a Mac (ALT key in Windows) then your adjustments will be slowed down. That is, the point will adjust at smaller increments when you move your mouse. This can really help when making fine adjustments to a point.
As you move the TAT cursor around your image you will see the corresponding point appear on the curve. This point will have a black center indicating that it has not yet been set on the curve. When you find the section of your image you want to adjust, click and drag the TAT. This will set the point on the curve and then adjust the curve as you drag. The point will now have a grey center and is fully adjustable directly on the curve as well.
Now. Should you decide that you no longer want a point you have already set you can delete it in several ways. Grab it and pull it outside the grid where the curve is shown. You'll see the curve snap back and the point will be gone.
Another way is to right click on the point and choose Delete Control Point from the contextual menu.
Should you decide that you want to start over and clear all the points you have added, right click anywhere on the grid and choose Flatten Curve from the contextual menu. This will remove all your points and return you to a standard flat curve.
Just as with the parametric curve, the point curve allows you to save curve presets. Once you have the curve you like, simply click on the drop down menu at the bottom of the tone curve section and choose Save...
As if all that isn't enough... you can switch back and forth between the point curve and the parametric curve by toggling the little curve icon in the lower right. The adjustments you make on one curve remain in place while you are in the other.
So you can switch to the point curve, make adjustments. Then switch back to the parametric curve and work on tonal regions. And then switch back to the point curve and adjust some more! The best of both curve technologies all rolled into one interface.
Also, presets that are saved in either curve mode are available in the other. So if you set up a point curve preset that you like to use as your starting point but you like to tweak the tonal values using the parametric curve then open the tone curve section. Make sure you are in the parametric curve display. Click the dropdown menu and choose your point curve preset. Boom! The parametric curve is adjusted according to the point curve preset and now you can tweak away.
This is a wonderful addition to the Develop module. And Lightroom 3 Beta 2 has still more to offer!