System Settings -> Window Management -> Task Switcher -> Main, click on the
Help button to open the corresponding help article. In it there's this:
Cover Switch and Flip Switch effect are more fancy effects which require OpenGL. Cover Switch displays the windows in a gallery with a large thumbnail of the currently selected window in the center of the screen while all other windows are rotated on the left and right. Flip Switch displays all windows on a 3D stack. The selected window is on top of the stack and navigating through the windows will move the stack so that the new selected window is on the top.
If the effect provides additional settings the configure button will be activated. By clicking this button a configuration dialog will be shown.
When compositing is not active or gets suspended the normal window list will be shown. There is no loss in functionality if an effect is selected and compositing is not active.
That means that one's machine should have OpenGL and that compositing should be active.
qdbus org.kde.KWin /KWin supportInformation | grep -E -A5 "^Compositing" would be informative.
If the output is like:
Compositing =========== Compositing is active Compositing Type: OpenGL OpenGL vendor string: Intel Open Source Technology Center OpenGL renderer string: Mesa DRI Intel(R) HD Graphics 520 (Skylake GT2) OpenGL version string: 3.0 Mesa 18.0.0-rc5 OpenGL platform interface: GLX OpenGL shading language version string: 1.30
both Flip Switch and Cover Switch should work.