Apologies if this was covered elsewhere but I didn't find it while doing a search in this section of the forums.
The concept is combination powers between teammates. There's a long history in games of buffs that various toons can grant others or acquire by working with others (protection, healing, leadership buffs to accuracy, etc), but what about non-buff/one off powers that also derive from teaming.
Example using Marvel: capt america tosses his shield and thor hammers it through a number of enemies drastically increasing the damage, or iron man shoots it and his repulsors divide thereby hitting multiple enemies/AoE.
How it would work in practice: During each fight a character can hit his 'combo toggle' (likely an option chosen at higher level to reflect proficiency/experience). As he cycles through attacks, the one that would be appropriate for a combo goes off and whichever other teammate can capitalize on it can light off his particular combo power to achieve the combo effect. This doesn't have to be a unique power unto itself, it can simply be one of the basic attacks/defenses the character would normally employ. Yet when teamed with a particular teammate it can be exploited to greater effect.
Example using CoH archetypes: i'm an archery blaster and i'm missioning with an electric blaster, or storm summoner. I click my combo toggle as it becomes available during the fight. I launch a fistful of arrows or rain of arrows. the electric blaster/storm summoner can then hit his combo toggle and fire off a lightning bolt that arcs across each opponent that has an arrow in them. This is just a crude example but hopefully it illustrates the concept.
It could also be 'practiced' to raise its effectiveness over time. For example, if you're constantly teamed with a particular group and your team often uses a combo power the amount of damage or number of opponents affected increases accordingly to some notional max. If you're randomly teaming, however, the combo would still work but effect fewer opponents and/or for less damage.
The combo could be set up as a chosen power/skill at a given level - opting for a boss killer vs mob killer type of approach for instance.
Although such combos would only work with certain powers and between certain power types where it makes sense, they would offer a further benefit to teaming and add a little spice to encounters. There are a number of console games that use this so it's by no means a novel idea. It does add a nice dynamic to teaming in an MMO though.