spells – Question about Guards and Wards confusion effect

The 6th level Sorcerer/Wizard spell Guards and Wards features a confusion effect, quoted from the text it states:

Where there are choices in direction—such as a corridor intersection or side passage—a minor confusion-type effect functions so as to make it 50% probable that intruders believe they are going in the opposite direction from the one they actually chose. This is an enchantment, mind-affecting effect. Saving Throw: None. Spell Resistance: Yes.

How is this implemented in the actual sense of players at the game table? Does the GM roll whenever the PCs make a choice at a corridor intersection and if the result is that they are confused, he or she states ‘you think you are going in the wrong direction.’ This seems odd as all this particular aspect of the spell seems to do is prompt a line of GM dialog whenever he is required to test and rolls 50 or under.

I would have thought it would have at least prompted an effect similar to the actual confusion spell where the test is made and if failed the player characters actually physically go in the wrong direction while still believing they are travelling in the direction they chose. But reading the spell description that isn’t what’s happening at all.