How do I replay games and guess the next move?

When reviewing a game, you can simply click on what you think is the next intersection played. If you are right the corresponding stone will appear. If not, you will be alerted by a beep.

How do I display and choose variations ?

The "Variations" submenu of the "View" menu gives you the ability to display the current move's alternative moves (siblings), or descendant moves (children). Alternative and descendant moves are displayed using a semi-transparent, black or white, icon. Clicking on these icons will chosse one branch or another. The "Move" info panel is updated to display the moves in the current branch.