This ability works as an improved version of a regular Sectoid'sMind Merge by granting the same bonuses (+25 Critical Chance, +25 Will, and 1 additional Health), but to all Sectoids in Line of Sight and the effects last the rest of the battle. The initiating Sectoid Commander gets no bonus. If the Sectoid Commander is killed, all the receiving units also perish.
This ability is rarely seen during the campaign since Sectoids largely disappear from the game by the time the first Sectoid Commander is encountered.
Greater Mind Merge's permanence is two-sided: it has the advantage of granting the critical hit and Will bonuses without requiring future turns for reapplications; but for the same reason the beneficiary units will not receive the slight health restoration which would come from reapplications; and the Sectoid Commander also becomes easier to locate since the psi-link from the receiving units can be followed back to the transmitting Sectoid Commander.
Don't get too eager about going for a multiple-kill: overextending your units could open up weak positioning that the enemy can exploit next turn. This advice is especially sound when playing in multiplayer, since your opponent may count on you tracking Mind Merged units to bait you into a trap.
The ability can boost Sectoids and Mechtoids at the same time.