If your motorcycle helmet doesn't stay on if you are tossed off the bike unexpectedly, it is of no use to you. Motorcycle helmet safety largely entails retention.
To check the helmet's retention capacity fasten the helmet strap snugly around your chin and then pull the rear of the helmet as if you were trying to lift it up and roll it forward off your head. Although you'll feel discomfort, the helmet should not come off with force.
Retention is more of a problem among half motorcycle helmets and open face helmets (also known as 3/4 motorcycle helmets).
For most helmets, the strap will tighten by passing through two D-rings. Despite attempts to manufacture quick fasten buckles and other forms of straps, the D-ring method is the most effective, convenient and attune to motorcycle safety.