What's a liver drag exactly?
 

We have plans to buy a dog in the nearby future and I try to learn somethings about using her for working outdoors.
Now i have read a few times about training a dog on a liver drag.
Whats exactly meant by that?
Just drag a bag with liver over the ground or something?
Should it leave a blood track or is it meant to leave a smell trail?
And when its mentioned to leave little 'snacks' along the track, should it be parts of the 'thing' thats dragged over the ground or can those be something else?
btw, we going for a wirehaired dachshund.