Watching casts I often see players bunching up their units in order to trap or block enemy commanders, the casters refer to it as "shift G" however by default this doesn't seem to do anything? and when I look through the hotkey list nothing jumps out at me.
Any tips would be appreciated.
It is called Spread attack, search for that and you'll find it
A work of art is never finished, merely abandoned
To do "shift-g" as a move order, you need to:
1 - select multiple units (the ones you want to bunch up)
2 - they need to have a regular move order. If they don't already have one, you can give them one
3 - then while they are still selected, hold "shift" and press "g"
You should see their move order change. Instead of one order for all of them, they each get their own individual move order. And this causes them to behave differently.