WebForm_OnSubmit() Js method is tied to form's submit event.
Here is what is inside WebForm_OnSubmit() -
function WebForm_OnSubmit() {
if (typeof(ValidatorOnSubmit) function && ValidatorOnSubmit() false) return false;
return true;
}
ValidatorOnSubmit() method will inturn call ValidatorCommonOnSubmit() if the control that caused postback have CausesValidation set.
ValidatorCommonOnSubmit() is the one responsibile to check if all the validators are valid.