Unlike most aircraft, the DC-8 is certified to use reverse thrust in flight (albeit only on its inboard engines). This serves as a very effective airbrake, although it also produces considerable buffeting (due to the large areas of separated and/or reversed airflow it produces over the wing’s surface).
[...] The pilots in this case, if the drive system failed during a PIO rather than earlier, had no way of knowing the real reason for its failure to operate in the ANU direction. In the split seconds available to them for analysis they could easily have concluded that the failure was due to heavy stick forces. Reverse thrust, in addition to drag, produces a nose up pitching moment, a fact known to the first officer if not to the captain, and as indicated previously, they did employ this aid. It is also true that during the time, no matter how short, required to go from forward thrust to reverse, the noseup pitching moment of forward thrust has been removed and therefore contributes to the severity of the dive. Small as it may be, this factor becomes more significant at very low initiating altitudes. [Page 25 of the aforementioned AAR; my emphasis.]
For an aircraft with engines mounted below the aircraft’s center of mass, like the DC-8, only the use of forward thrust would be expected to produce a pitch-up moment (due to the torque exerted by the offset net thrustline), and reverse thrust should produce a pitch-down moment (for the same reason):
Yet, for the DC-8, the use of reverse thrust not only creates a nose-up pitching moment, but, apparently, creates one that is stronger than that produced by forward thrust!