As explained in Help text;
In other words; when the Ctrl-key (or Cmd-key for Mac) is held down whilst using the mouse to click+hold within the HSL tool’s selected color-range, a mask is applied to the image - with all colours outside that range being shown in grey-scale … which shows exactly which colours in the image will be affected.
Additionally, with PLv6 (but no longer with PLv7);
Holding down the Ctrl-key (or Cmd-key for Mac) whilst moving any of the colour-range handles results in the mask being applied interactively - reflecting changes made to the colour-range in real-time.
This enables very precise finetuning of the colour-range.
However, this interactive behaviour no longer works with PLv7 … It’s now a tedious process of adjusting the colour-range, releasing the mouse (from it’s click+hold on one of the colour-range handles), re-applying click+hold within the selected color-range, evaluating the result … Repeat.
Curious !! … This degradation in PLv7 (c/w PLv6) is similar to the loss of interactive feedback when placing a Local Adjustment’s U-Point … as reported here.