What is the beam the Sectoids shoot at each other?

Solution 1:

The ability is called Mind Merge.

As long as the beam is maintained, the target sectoid gains these benefits:

  1. One additional health bar (meaning they can no longer be killed with 1 grenade).
  2. +25% crit chance

If you kill the sectoid giving the mind merge, the receiving unit will immediately die. The mind merge effect takes precedence over explosive damage death, so if you catch two sectoids (one mind merging the other) in a rocket, you'll still recover some weapon fragments.