What is the purpose of valve overlap?

Answer 1

Overlap allows the exiting exhaust gases to help pull the intake air/fuel charge into the cylinder. This increases volumetric efficiency/cylinder filling especially at lower rpms, when intake charge velocity is low (torque builds quicker). More overlap/Narrower LSA(lobe separation angle) usually results in a peaker powerband/higher Tq/Hp #s. Downsides are: some bled-off cylinder pressure, contamination intake charge(part throttle especially), weaker vacuum signal, rougher idle, narrow powerband, poor part throttle running and higher emissions due to both valves being open.