The Debug/Expert mode gives you more buttons and deeper insights into special parts of the UI. 

In 2sxc 11 - 13

Most dialogs don't have hidden features any more, because most advanced features are now visible to all users. But you'll still see some advanced features in the edit-dialog or in  App > Data. 

• To enter or exit Debug, hit Ctrl+Alt+Shift+Click anywhere on the dialog
• On Mac, use Ctrl + Shift + Option

You should then see a hint that it was enabled. In some dialogs you'll then also see new buttons or features: 

In 2sxc 10 and before

Only dialogs that have the "π" character in the corner (homage to a famous sci-fi movie) have the debug/expert mode - it looks like this - and the mouseover will show you more information about the system: 

In 2sxc v.4.0 - 10.25 and the old Edit-Dialog

• To enter or exit it, just hold down the Ctrl button and click anywhere in the UI. You should now see more buttons (usually orange) and possibly more things in the dialog. 

In 2sxc 10.25+ (the new Angular Dialogs)

• Use Ctrl+Alt+Shift+Click to enter or exit debug