Feeding tubes were demonstrated to me in this way: Use a glass of water and a
straw. Fill you mouth with water then try to suck water up from the glass out
of the straw. You can't get suction. If the baby's mouth is filled with
breastmilk, they can't get suction on the tube. Of course if the tube system
is too high or if  you get a gravity feed (as with one type), the suckle gets
lazy.
I can't remember where I read it (Susan's Morris maybe?): that the presense
of fluid makes the suckle more efficient. The stronger suckle may make mom's
let-down work better too.
I 'm not sure if anybody knows why or how they work.
Marie Davis