Hi Philippe,
The feature is "HTTP Firmware Upgrade" which is a feature of the 9.x phone firmware.
please check the below release notes for the same,
http://www.cisco.com/en/US/docs/voice_ip_comm/cuipph/7900_series/firmware/9_3_1/release_notes/P790_BK_RF1DAED3_00_rn-9_3_1-7900-series.html#P790_RF_H0DA5A91_00
according to this, the prefered method is http download and if that fails it will fallback to tftp download.
Please note that the phone will try to create TCP socket at port "6970" and if that is blocked then after some retries the phone will proceed for the normal TFTP download and because of this you will experience delay in phone reset and extension mobility login and logouts.
Thanks,
/Aashish