What is 'HIP'?

How about this then?

1. Take LHR-DME as a separate trip
2. Start a RTW from LHR or CAI, do all my stops on my way to WLG, then forget about the WLG-LHR 'return' journey'.

In terms of 'back-tracking', from what I understand, it seems that as soon as you've left a continent, you can't use it again, not even for transit. For example, if I depart North America to South America, I can't transit North America to get to Asia?

The agent said 'no', but posts here said it can be done?