Difference between [(ngModel)] and [ngModel] for binding state to property?

Question

[(ngModel)]="overRideRate" is the short form of [ngModel]="overRideRate" (ngModelChange)="overRideRate = $event"

[ngModel]="overRideRate" is to bind overRideRate to the input.value
(ngModelChange)="overRideRate = $event" is to update overRideRate with the value of input.value when the change event was emitted.

Together they are what Angular2 provides for two-way binding.