When is the Redo-port called with new variables in Trace/0 and when not?

Question

During my implementation of the Adventure game in Prolog I was wondering when the redo-port is called with new variables during backtracking and when it is called with the s

Answer 1

This is just a comment posted as a answer because it has a picture.

If you use the graphical debugger you can see when choice points are created.

?- gtrace.
true.

[trace] ?- (location(X,Y),door(kitchen,Y)).

In the following image of the debugger I highlighted the choice point with a green rectangle.

To exit out of trace mode in top-level enter nodebug.

[trace] ?- nodebug.
true.

?- 

Note: It is the choice points that lead to using the redo port.
Note: Another way to debug prolog is via use of failure slice. Also read questions tagged with failure slice