Mr. Optimistic asked for an explanation of the term "downwash". I refer to the use which DPD makes of the term in his book. I think it is quite clear what DPD means by the terms he uses.
"Incidence" is denoted alpha, and as mattpilot has noted, "incidence" and "angle of attack" are explicitly the same for DPD (quote from p4): the angle between wing chord line and free air stream. Free stream is defined as the air conditions close to but uninfluenced by the airplane. Not all people nowadays use exactly these definitions.
The airstream is turned downwards for various reasons after passing over the wing. The tail sits in the downwards-turned airstream, which is for the tail its "free" airstream. DPD uses "epsilon" for the angle of downwards-turning, aka downwash. Let alpha_0 be the angle between the wing chord and tail chord. Then the incidence (=angle of attack) of the tail is (alpha - epsilon) + alpha_0. This is all on p120. This is all pre-stall terminology.
Considering the term to use for a superstall, DPD says "Any title, such as super-stall, deep stall or a locked-in condition, is acceptable because they all refer broadly to the same thing; the only point to be made is that the locked-in description should be applied only to those types on which recovery is not possible", p115.
He says that there are two characteristics which affect whether an airplane has a superstall, namely the pitching tendency of the airplane at the stall, and tail effectiveness in recovery from the stall.
First, some swept-wing airplanes have a pitch-up tendency at stall. This may be due to a number of factors: if tips stall first in a swept-wing aircraft, then center of lift (CoL) moves forward, and thereby closer to CoG, reducing any nose-down moment caused by weight x arm (measured to CoL). Further, the lift generated by the forward fuselage of the aircraft usually continues to increase until well past the stall, contributing to the pitching-up tendency past the stall (I presume he is presuming that the lift generated by the fuselage forward of the -changing- CoL is greater than that generated by the fuselage rear of the CoL, even as the CoL is moving). Third, the aircraft will start to sink significantly, increasing alpha even without change of pitch angle. This "compounds the entry to, and the progression of, the super-stall."
If the tail then sits - quite quickly, as inboard airflow separates - in turbulent airflowthe from the stalled wing rather than its usual free airstream, then elevator effectiveness is - quite quickly - much reduced. In some aircraft, the tail is positioned such that this can happen. And if you are in a high-alpha position with much reduced elevator effectivess, it can be quite difficult to impossible to get out of this situation.
DPD is, as John Farley says, referring to a very specific form of behavior of some commercial swept-wing jets at the unaccelerated stall when he talks about "super-stall".
I note that current airworthiness certification requires that it be demonstrated that a commercial aircraft be not able to enter a superstall. Aircraft whose aerodynamics and geometry render them possibly susceptible to one must be fitted with automatical devices which preclude it (stick pushers).
PBL