How would heart function be affected if the AV valves didn't completely close during ventricular contraction? What problems would the backflow create?
It can cause a problem called regurgitation.
Regurgitation is a problem in which your blood flow is opposite than normal, which can lead to various heart problems, such as MVP (Mitral Valve Prolapse). It is a problem because your heart is like a pump, and everything that needs to be sealed is sealed in the right time, so if your AV valve is affected it won't be sealed like it was before, which can lead to the return of the blood, and therefor to an abnormal heart rhythm, palpitations, shortness of breath...