Count down method is used to get the jig at a certain depth BEFORE you start the retrieve. The speed of the retrieve, at that point, is what keeps the jig at the required depth .... well, to a degree, more or less.

If I cast a 1/16oz jig/plastics 40ft & start my retrieve at splashdown, hold my rod at about the 10 o:clock position, & reel in just fast enough to keep a slight bow in the line (constantly thru the retrieve) ... I'm confident that the jig will reach around 8ft deep at the deepest portion of its dive. If I want the jig to basically stay at the 8ft depth level thru the majority of the length of the cast, I'd count it down (8secs) at splashdown & lower my rod to about the 8 o:clock position & speed up my retrieve to where the line didn't have much (if any) bow in it. I wouldn't know "for sure" that the jig maintained the 8ft depth throughout the majority of the distance, but I could stop reeling when the line was getting close to being slightly angled down from the rod tip & lift the rod up and gauge how deep the jig was at that point. Or, as most people usually do, I could always make several casts to the same spot & countdown starting at say 6secs .... and progress thru the countdown all the way to say 10secs .... and figure I probably ran my jig "close enough, long enough" thru the depth range at some point in that session of casts.