ETags (“If-None-Match”) provide a simple method of saving bandwidth, by checking whether the rates have been updated since your last request.
If the data have not changed since your previous request, you can fallback to a cached copy, and your API response will be under 0.2kb (instead of the standard 2–4kb response). If the rates have changed, you'll receive the latest data as usual.
It's easier than it sounds - here's a step-by-step guide to prove it!