Stripping the ZF4 HP18
Part 1    • Investigating failure of a ZF4 HP18 T...  
Part 2    • Investigating failure of a ZF4 HP18 t...  
Part 3    • Investigating failure of a ZF4 HP18 t...  
Part 4    • Investigating failure of a ZF4 HP18 t...  
Part 5    • Investigating failure of a ZF4HP18 tr...  
Inside an automatic gearbox pump (ZF4 HP18 frontwheel drive)
   • Inside an automatic gearbox pump  

Quick vid showing how the pump works.
Clever design means it's compact so it fits between main body of the gearbox and the torque converter. As the engine runs, the torque converter (which is bolted to the flexplate/flywheel) spins. Part of the converter has a notched extension which engages with the inner ring gear on the pump which spins it and causes the pressure difference across the pump thus causing flow of ATF under pressure to the valve block and gearbox.
Due to the finely machined surfaces, clean ATF and regular oil changes is a must. Dirty oil will accelerate wear due to the fine metallic particles slowly eroding away the machined surfaces which eventually leads to drop in system pressure.
If you watched the previous ZF4 HP18 videos, you would have seen the colour of the contaminated oil which was grey with metallic debris instead of a deep ruby red colour.