Bruce Lee described this as being formless, but only now am I really understanding the depth of it. So often I've found myself consciously determining action, "they said this, now I will counter with this because..." or "I will do this to elicit this response, which will allow me to...". It's too much thought and interrupts flow. That level of planning and thinking is important but it can also hinder creativity and true expression.
I used to see this a lot when I was practicing judo, you learn a new throw and then you look for every opportunity to use it in competition. While mastery of technique and recognition of opportunity for proper application is important, that narrow focus will cloud the recognition of other opportunities that will achieve the same desired result. Maybe instead of waiting for your opponent to be in position to apply that throw, he's already in position for osoto-gari, but you don't see it because you're sitting there waiting for tai-otoshi to show up and thus miss it.
I also used to see it in jazz improvisation a lot, you learn a new lick (maybe some cliche diminished scale lick like at the 2:06 mark here), you can't wait to use it and you look for every opportunity during improvisation, maybe forcing it in there as if to say "look what I learned!" Compare to this to the approach of someone like Michael Brecker, who once said in an interview that when he practices a lick or line, it might be months before the line comes out naturally in his improvisation, and even then it might come out in a completely different form.
Instinct with true expression, versus consciously and arbitrarily forcing a framework. That's the essence of formlessness.