LOCK means you need to press and hold the LEFT mouse button to adjust a setting, the button you are locked onto appears in (annoying) blue color and a box appears with a description of the button and the current setting (for example selected altitude or speed), you then change the setting by moving your mouse. I believe hovering over a button and then scrolling still works when using LOCK. The annoying thing is that with some simple functions, like clicking the heading button to synch the heading bug now required you to first lock on to the button (press and hold left mouse button) followed by pressing the right mouse button to synch the heading. You can get used to all of that, and it is useful in turbulence to be able to “hold onto” a switch. My main problems with it are the stupid blue highlighting of whatever you are locked onto and the big box with instructions appearing which blocks the view of whatever is behind it, you can mitigate this somewhat by changing the UI size.
Legacy is what you are used to.