As Roy says, you activate the Shadow & Highlight indicators from the VIEW menu and you can set the activation values in Preferences. The purpose of this tool is to warn you when you are cutting out information that is represented by 'peaks' in the histograms that are too small to register on the diplayed scale, yet may be of importance in a particular image. See CambridgeinColour. In a process like DPP editing where RAW editing and 'converted' editing (usingthe RGB pane) are combined the tool also warns you that you have squeezed the conversion curve to a dangerous extent, in this case the value is set by DPP to correspond to what you have done in the RAW pane. Probably why it is designed not to be left on with preset values.

Squaring the curves (and also changing from RGB to Luminance) is done when setting a point on any curve in the RGB pane; pressing control at same time as creating point shows the sub-menu.

Someone like Robert (Robski) or Sean (of CinC) understands the science of all this and is often kind enough to explain the rudiments to us lesser mortals. However the beauty of DPP, at least to Mac users, and other intuitive types yet to discover Mac, is that you can just play intuitively and get instant feedback of the results of what you are doing because the live preview is so good and immediate. Ideally, if one has the exposure etc correct, it is only necessary to choose the best RAW conversion curve or just leave the standard one. Shooting anything with very wide EV range like landscape, a bit more tweaking in RAW is required. Only when desperate does one resort to the RGB pane using 'curves' techniques to tease out something you know was there and hope the box has caught in its net if only you can find it. I only touch levels and anything other than curves in the RGB pane when re-editing .jpg from older E4500 and FZ7 images. It is brilliant for the latter for the simple reason that it is all done and dispayed concurrently rather than sequentially and battling with levels as in PS(E).

The danger of 'overcooking' frequently emanating from PS addicts is also present in the RGB pane of DPP.