The Hornets were in trouble at half-time after poor defending had allowed Andrea Orlandi and Steve Davies to put the basement side two goals up at the interval.
But once Odion Ighalo had reduced the deficit within two minutes of the restart, the Hornets took complete control as the Nigerian striker netted three more times and Troy Deeney, Matej Vydra and Gabriele Angella all scored in a stunning turnaround.
Asked what he had said at the interval, Jokanvic replied: "Nothing. At half-time we decided to put one midfielder more on and play with one defender less. I didn't speak so much at half-time.
"Really I didn't speak so much - but I believed we could change our situation.
"We scored many goals, but what's more important is probably the first goal that we scored (two minutes into the second half)."
Blackpool manager Lee Clark said afterwards: "At 10 to four it was looking fantastic. At 10 to five it was embarrassing. That's quite simply the easiest way to put it.
"We talked about the first 15 minutes (of the second half) being crucial and not conceding, certainly not conceding three in the first seven or eight. They put us on the back foot and we never recovered.
"I praised them at half-time, but I told them that the first 15-20 minutes was crucial.
"We had to be solid, we didn't have to be gung-ho, we didn't want create spaces for the attacking prowess of Watford but within minutes that had changed. We were still 2-1 up but we didn't handle the setback well enough. It just went from bad to worse.
"We showed we can do it in the first half - but you can't play like we did in the second half, at any level."