[yetihehe]

Phone detects that you call emergency service and enables gps.

Last time I called 911 (well, it's 112 in my country) my android phone asked if I want to provide gps coordinates. I did, but they still asked for address, so probably this is not integrated/used everywhere.

[nkrisc]

They may also ask simply to confirm the location is correct and to help responders more quickly locate you in the vicinity.