For me people feeling the sand paper effect, i.e. the barbels' barbules feeling the bait without indication on the tip is a total fallacy. Assuming the bait is on a 2.5-4ft hook length typical of most feeder rigs the bait will be downstream of the feeder. Fish on a river tend to face upstream, letting the current bring food to them unless they are actively feeding in which case the search the bottom moving upstream as they methodically search the bottom.
If you watch a number of the on-line video's of fish feeding it's a well know fact that a fish can mouth and eject a hook bait multiple times, even on the most sensitive bolt rigs/sets ups without the angler being aware of the activity. The first thing an angler will know about a bite is when the fish has the bait in its mouth and moves sideways or downstream. This is indicated by either a knock or pull on the rod tip or by a drop back bite if the feeder is dislodged.
If the fish moves directly forward with the bait in it's mouth this will give slack between the feeder/lead and the bait and as such would cause no indication leaving the angler unaware of the activity until the slack in the hook length is taken up and the fish either moves sideways, forward past the feeder or drops back down with the bait in its mouth.
Twitches, pulls and knocks are generally caused by either a fish having the bait in it's mouth and moving away with that bait, small fish plucking at the bait, fish brushing the line or fish moving the feeder. The only other thing is debris or weed brushing or hitting the line between the rod tip and feeder/lead while the line is under tension.
Most phenomenon's can be explained however you need to look closely at the physics, methodology and the information proven by science, (in this case video footage), to try and understand what is actually going on underwater and how it all links together. If you haven't seen the video's of fish feeding and the impact of rigs, sets ups they can be quite enlightening and are well worth watching.
This is a stillwater video link, the difference being that on a river the flow will actually straighten out the hook length so that it is downstream, however if you watch the other submerged video's it may not be behaving how you think it is.