I also think that the term is cheesy.
And of course its in your head. Everything is in your head.
Your knowledge, your perception, your pattern recognition, decision making skills and logic, your emotions and your fears, everything you do- except for involuntary bodily functions-comes from your head.
However, the trance-like state that people commonly refer to as deadstroke is real.
Because your head is very similar to your muscles in the context of being able to be trained. You can train yourself to think in "left brain" mode or " right brain" mode. You can train yourself to overcome fear and emotion. You can train yourself to concentrate in distracting circumstances and environments.
The trick, or goal, as the case may be is being able to recognize and understand your current state of mind, and adjust accordingly at any given point in time. Which by the way, is MUCH easier said than done. If it were easy, everyone could and would do it.
There is a physical mode of being "in stroke" and a psychological mode of being in stroke. And while its true that if you have no skill to speak of, your state of mind will make no difference, but at the same time you can be the most skilled player on earth and if your mind is in the wrong place, you won't shoot worth a damn.
For most folks the dead stroke condition is a total mystery simply because they don't know what they did to get into that particular frame of mind and can't seem to conjure it up at will, because, like I said, it is much easier said than done. Artists, musicians, writers , and creative types in many realms can all relate to this conundrum of 'inspiration-on-demand' and generally speaking, will all agree that it takes work and practice to achieve.