The concept of "Flow" is a state people can enter when they are completely immersed in their work, fully engaged at such a level that they do not notice the passage of time. This is how you can look up from a good book and realize it's after midnight, or why kids always seem so surprised to see their parents come in after their two hours of game time is up. "But I only played for an hour!" they might say. (and when they do, a game designer gets their wings!)
Jenova Chen (Creative Director at thatgamecompany, makers of Journey) has a great primer on Flow, if you want to do some more reading on the subject.
When it comes to games, a team seeking to put their players into "flow" must do everything they can to not interrupt play. This includes loading screens, long cutscenes, dropped frames, and other glitches that basically scream "THIS IS NOT A REAL EXPERIENCE."
In the first stage of its development, every screen in Chess Heroes was its own level. This made it extremely easy to prototype quickly, and altering the "flow" (lower-case f!) through the menus and into the game and back out again was easy, too. But! Now that the game is getting into a more final state, I have to reduce the number of level loads so that "Flow" is maintained. This means smooth transitions between menu states, and clever ways to hide the frozen state of a game loading the next level. As you might expect, this is taking a bit of time, but with the look and "flow" finalized, it's worth it to finally nail down the "feel" of the game. This starts with the menus, flows into gameplay, then pulls back out again. So far, it's been a treat, and I'm ecstatic to be working on a fully realized polish pass, instead of hastily rushing through with a "good enough ok ship it" mentality.
And yes, when they say the last 10% of the work takes 90% of the time, they weren't joking. It really burns through the working hours, but at the end of the day, it's absolutely worth it.