The research subject you're looking for is the Charged Missiles ability. Requisite research is both the Kanrak and the Junsurak, the subject itself is Tier 4 Military.
It's effects on the Sentinel are the same as those on the Assailant - when it hits a target, other nearby targets will get hit by pulse beams originating from the first target. It's one way to enable the Sentinel to damage multiple targets with each phase missile it launches.
