Half back is one half of the critical area, the other is having a quality sniffer like McCaw.
If you want your forwards to get momentum you must secure the ball at the ruck quickly and nobody does that like a quality sniffer.
McCaw is ancient but he's still the King of the sniffers, Pocock was awesome for a while for Aussie, Broussow was excellent for the staffers until he went out of favour, Neil back was brilliant for England.
You need someone quick and agile with no particular care for their personal well being to get into areas 'quickly' and contest for the ball, they must sniff it out.
That problem resolved the other key element in the mixture is the halfback must pass the pill on QUICKLY.
We used to have a bloke at halfback for more years than i care to remember, his name was Justin Marshall, he ALWAYS took a step before passing the ball either way and it telegraphed everything the All Blacks back line were going to do, or which forward was going to carry. He was internationally renowned and lauded and the truth is he was slow and rubbish and we never won a world cup with his slow delivery at the base of the ruck.
THAT is how vital quick ball from the halfback truly is...
Get a sniffer of quality to secure the ball quickly, get some good hard running forwards to blow the enemy out of the ruck pronto and a halfback who shifts the ball IMMEDIATELY (preferably accurately) and ANY forward runners and ANY back line will ignite.
England circa 2003 had Neil Back and Richard Hill securing and blowing out and Dawson chucking it at pace, look what happened...
1987 the AB's had Michael Jones sniffing, Alan Whetton blowing out the rucks and David Kirk chucking...
2011- well you know who was there
The saffers were dangerous when Joost van der westhuizen was on form...
A young Mike Phillips (gosh that seems a long time ago) behind the ginger sniffer...
These combinations are what get the forward ball carriers on the attack pronto and that means hard questions for the defenders.
This opens up opportunities for the roll on and then the attacking teams start to smell blood in the water and some capitalise and others get white line fever and blow a gasket.
Bottom line is, no sniffer and no seriously quick half back then no momentum.