Sniff subrating is a Bluetooth feature designed to increase battery life as much as 500 percent for devices whose typical usage involves a significant amount of inactive time. Such devices include keyboards, mice, headsets, and home sensor networks. Sniff subrating was introduced as part of the Bluetooth Core Specification Version 2.1 + EDR (Enhanced Data Rate).
Sniff subrating enables Bluetooth 2.1 devices to decide among themselves how long they will wait before sending keepalive messages to one another. Previous Bluetooth implementations featured keepalive message frequencies of up to several times per second. In contrast, the 2.1 specification allows pairs of devices to negotiate this value between them to as infrequently as once every 5 or 10 seconds. Because keepalive message exchanges often comprise the vast majority of activity for devices that are frequently idle, this technique extends their battery life significantly.
Existing Bluetooth devices that predate the specification release (mid-2007) cannot benefit from the 2.1 + EDR specification. Only devices that include circuitry developed to comply with the 2.1 +EDR specification will be able to take advantage of sniff subrating (and other features) to extend battery life as described here.