Gnome-Pie is activated by keyboard shortcuts. You press on the keyboard a predetermined combination, such as Ctrl+Alt+A (seemed to me more convenient keyboard shortcut Alt+Space) and the screen on top shows icons located around the current mouse pointer position. When you hover the mouse over the icon appears the name of the action that will be executed after clicking on the icon. Each icon can serve for different purposes: to launch an application, open a folder, close the window, shut down your computer and so on.
All buttons can be configured. You can create multiple groups (pies) and ask each group their combination on which this group will be opened. For example, you can create a group "Games" and put shortcuts of your favorite games and assign, for example, Ctrl+Alt+G on which will appear the menu for quickly launching your favorite games.
Installation latest version into Linux Mint:
sudo add-apt-repository ppa:simonschneegans/testing sudo apt update sudo apt install gnome-pie
All of your settings in the window Gnome Pie - Settings. After starting Gnome-Pie in the taskbar (system tray) icon, clicking on which you can open the program Settings. In the tab Pie you can view all the keyboard shortcuts to open "pies".
So the program starts automatically at system startup, in the General tab, set the mark Startup on Login.
Gnome-Pie supports different themes. On the whole, the program performed very solidly, although it is developed by just one man.
"Pies" can only run apps/execute commands and to facilitate work with many combinations of hot keys, but to perform some predefined action when the application starts Gnome-Pie is not able. In this regard, Gnome-Pie is clearly not up to Synapse or Kupfer.
Remove from Linux Mint:
sudo apt-get remove --purge gnome-pie