I think you've just been lucky. In the case you describe, the combination of the downhill and the use of only the rear brake probably caused the skid you described. During deceleration your weight will shift naturally to the front wheel, unweighting the rear and making it much easier for the rear wheel to lock up and skid. That you were going downhill may have contributed to the added weight on the front wheel, exacerbating the problem of the rear skid.

For more efficient braking, especially in an emergency, you need to use the front brake and shift your weight back. Shifting your weight back weights the rear wheel so it is less likely to lock up when you apply the rear brake and will help you get over the uneasy feeling that you are going to pitch forward over the bars