I hear a lot of discussion about using end of session signals and I’ve heard them described as jackpots as well.
End of session signals and jackpots are controversial and there is not a lot of science backing up their use.
There is lots of reasons not to use them, such as using a signal indicating a lack of opportunity for reinforcement potentially being a Time Out or NRM (No Reward Marker) Punisher. Who wants to punish the behaviour you’ve just spent a whole session reinforcing?
Similarly, jackpots lie in the realm of gambling vernacular. It’s a very large and surprising windfall. In order for a jackpot to have any impact on behaviour, it needs to be contingent and delivered contiguously. This means it needs to be delivered dependent on a certain behaviour being performed and delivered in the very moment it was performed. So why would we deliver a jackpot at the end of a training session? If we do it every time, we become predictable, the animal expects it and they can potentially end up looking forward to the end of the session (where they get a windfall of reinforcers) more than the actual session!
I had an interesting and surprising conversation with Ken Ramirez about end of session signals many years ago. It had been drilled into me that we must use them! But Ken was of the opinion that they were not that important and potentially punishing. When we finished sessions with his goats, donkeys and alpacas, we sometimes dumped food and exited quickly with the goats (they are fast), or switched to scratches with the donkeys, or with the alpacas, they were still ambivalent about people and food and our departure appeared neither here nor there. Although we always left a parting gift to keep them busy and distracted when we left their area.
So although a clear cue may not be recommended, leaving some food, a puzzle, a distraction, is still a good idea. We don’t want an animal chasing us to the gate and pawing or or getting frustrated as we walk away with food.
Sometimes a cue may not be what we intend either. For example when I train more than one animal in a row, my verbal end of session cue for one animal, is heard by the next animal in line. For them, it is a start of session cue and I can hear them verbalise their excitement that it’s their turn next!! 
I have a cheap doormat I like to use, as a station. It also has little dimples and is great for trapping chaff or pellets. I sprinkle some on it and it becomes a fun and enriching game for the animal to hoover up all the little crumbs. Meanwhile I have departed and they are happy and busy and I haven’t had to tell them the fun is over for now.
Check out my video of Seymour the donkey enjoying every last crumb that I’ve sprinkled on his mat: