[Suicide Junkie]

Shift clicked fleets will move together to the same places, but not neccesarily at exactly the same moment. I did some tests with it and could never get them to attack at the same time, regardless of varying fleet speed.

I used it to attack a super-heavyweight defended planet in simultaneous (Play by LAN). I shift-clicked to send in an unfleeted Battlemoon, a fleet of support ships, and two 120 ship fleets.
Result: one combat, with 250 ships spread across the entire right half of the combat map.

About five ships shot at the planet (since they started in range), and a transport decided to retreat to the far corner. As it passed the planet, it decided to drop its troops as well.
The fleet stopped firing at the 30-odd massive platforms since my troops were on the surface, and my ships got ripped to shreds.

The only survivors were: the battlemoon, the minesweepers and repair ships, and the troops on the surface.