I've a feeling there may be no easy answer to this one, but here goes. The issue is how to ensure the feed, in river fishing, is where the bait is. In a shallow, slow river it's no problem because, even if the feed leaves the feeder when it smacks the surface, it'll drop near the hook bait. But what can you do in a deep fast river?
When does the feed leave the feeder? I can imagine it happening the moment it hits the surface. Or maybe all the way as it descends. Or maybe not till it hits the bottom, in which case I assume it will stay in the feeder till you strike.
I know part of the answer is that your feed has to have the right consistency, but how do you know whether this is achieved? You wind in. If the feeder is still clogged up with feed you know it's too hard. But if it's empty it could be just right, or it could be it's far too dry and fluffy and has ended up yards downstream of your bait.
Any advice would be welcome.