"Awareness of the flow" doesn't seem to be a standard phrase (if you Google for it). So I tried two Google searches to try to find out how he (Stephen Levine) uses the phrase (I haven't read his book):
Two of the quotes or paraphrases I found by doing this are:
He says everyone must start by focusing on the mind, as an observer, to watch thoughts, as they pass through the mind, and to begin to recognize the constant change, and flow, that is present, from moment, to moment. He warns, not to let one's self get pulled in, or get attached to any one thought, but to detach and let each moment unfold.
When we attend to the ongoing mind we see that even "the watcher" becomes part of the flow. The who that's asking "Who's watching?" is another thought-flash we see go by; there's "no one" watching, there's just awareness. When the "I" becomes just something else observed in the flow we see we're not different from anything else in the universe.
These seem to be using 'flow' to refer to a 'flow of thoughts', a sequence of thought, a progression or an ebb and flow of mental process ... watching flow is the same as watching thoughts.
This ("flow of mental observations") may or may not be exactly the same as "flow of arising and passing away of all conditioned things" ... instead maybe it's "flow of arising and passing away of mental awareness of all conditioned things".
On the other hand I don't know how much emphasis you want to put on the difference between "the observed" (e.g. "all conditioned things") versus "the observation" (e.g. the mental awareness).
I think investigating the difference or relationship between observed and observation, between objective and subjective, has been a big theme of western philosophy ... it may or may not be important to understanding Buddhism.
There's also this quote from http://www.personaltransformation.com/stephen_levine.html
When it's my cancer, I'm alone with my cancer. I have nobody, just me and my cancer, and it looks like there is no way out. When I realize it isn't my cancer, but the cancer, there is space to work on it. My depression, my cancer, crams me in, like being in a phone booth full of my life. You open the phone booth and see that everybody is standing outside of their phone booth. It's isn't my depression, it's the depression. When it's the cancer, I am in this flow of human-kind, with all the energy of four or five million other people going through the same thing at this same moment. You connect to something universal, which brings peace.
Here he may be using the word 'flow' to refer to "conditioned things" (i.e. human kind), or he might still be using it to refer to 'mental process' (i.e. saying "I am in the flow of mental process that's common to human kind").
When you quoted, "what separation from the flow feels like", that's not clear or is a bit ambiguous. I think that phrase implies that "flow" refers to observation rather than observed (maybe we don't separate from a flow of things, separate from human kind, when we meditate, but might separate somehow from flow of observations). Assuming that flow is referring to the flow of mental observations (and assuming it's even meaningful to distinguish that from the flow of observable/conditioned things), I think that "separation" might mean either suspension of observation (e.g. entering a jhana state), or a disassociation or dis-identification (i.e. some kind of awareness of anatta).
