When you try to get the car dealer to finance a car the dealer may let you drive the car off the lot without the financing being approved. This is called spot delivery. Then a week or so later the dealer calls and tells you the financing was not approved and, of course, if you had a trade-in it is gone and cannot be returned because it has already been sold to the wholesaler.
So now you have two choices. Either pay more for the car in extra costs and fees or return the car to the dealer, hence the name yo yo financing and it is all legal because you drove the car off the lot without being approved for the financing.
If you do not want to fall prey to yo yo financing do not sign the form the dealer gives you that says you agree to take the car off the lot without the financing first being approved. However, not all dealers that have been guilty of yo yo financing have secured a signature on the form from the car buyer so even if you do not sign this form you could still fall prey to yo yo financing from the car dealer.
SUPPLEMENTAL SOURCES: ABC NEWS and AOL AUTOS STAFF 8/28/13