airbushelicopters.com
I hesitate too make this post as too many people want to ridicule and scoff.
But here goes one more time, as a very average Pilot, but some would say a pretty good Engineer!:
If you take the time to read page 5 of the above link you will see that EC know very well about the anomaly of fuel transferring from main tank to supply tanks after transfer pumps have been switched off.
There is no magic or witchcraft at work, just simple fluid dynamics.
The flexible transfer pipes are about 15mm dia and are the [U]only[U]path between main and supply tanks once the fuel level is below the fence transfer ports.
If the head of fuel is large enough, then after the transfer pumps are selected off the syphoning effect will continue pulling fuel through (for want of a better word) a freewheeling pump for several minutes.
The 4 submersible pumps are identical apart from the main tank pumps have NRV fitted, the supply tank pumps do not. There is a very good reason for this, the supply tank feed to the engines is non critical in respect of fuel drain back, the transfer pumps however are critical hence the fitting of NRV/check valves.
Now for goodness sake don't take my word for it, go and ask your Chief Engineers!