How do the ‘instant cash’ offers work?

Suppose this is more of a general question, since I’m new to freecash. Just got an offer that said something like ‘spend another $15 and get $20.31 back instantly’!

Obviously there’s a catch so just curious what that is and how it works. If I spend another $15 to take me to the second bonus tier, will I actually have that much extra in banked awards, or is the wording misleading and it’s more just what I ‘could’ get in upcoming rewards or adding on to stuff I’ve already banked?

Thanks! Freecash ID: 123456789