What is a Dropback?

I got out carp fishing this evening and landed 4 mirrors.  Strange thing is two of these fish were hits that were dropbacks.  So, what is a dropback?  This is simply when the line suddenly turns slack and comes in.  When that happens, the fish is actually swimming toward you with the bait and hook in its mouth..  If I get a dropback, I usually pick up the rod and reel in the slack.  If it keeps coming toward me, I usually just pull and hook the fish.  However, sometimes the dropback just comes back a foot or two and the fish drops the offering.  In that case, I reel in the slack and reset the rod and reel back on the alarm hoping the carp will return.  Dropbacks are fairly common occurrences in carp fishing.