The 6.8 True Velocity shows it pretty well:

Rather then having a 'neck' it uses polymer to make the shoulder of the cartridge into a block of plastic with a hole in it, which serves as a sort of 'internal neck.' 

This can be used to either increase ogive space, or increase powder capacity. Or in the 6.8's case, seems to be a bit of both. 

The added advantage is that it makes the polymer cartridge much more durable. Rather then the bullet being held in place by ~1mm or less of plastic, the bullet is embedded in a ~10mm circle of solid plastic. 

Thank you! This is interesting.

The idea goes back quite a long way. The 25 x 145 shown fourth from the left below was made by AAI in 1973.

Is this technology now considered mature?

And does anybody know how thick the walls are?