What maneuver is prohibited with the PMU switched off?

The correct answer highlights that spins are specifically prohibited when the PMU (Power Management Unit) is switched off. The PMU is critical to the operation of the T-6, managing engine performance and ensuring stability in various flight conditions. When the PMU is off, the aircraft loses its automatic functions that help prevent adverse situations, such as spins.

In normal operations, the PMU assists in maintaining optimal engine performance and provides critical protection to prevent scenarios that could lead to loss of control. In the event of a spin, the aircraft's controllability and recovery procedures are highly reliant on having the PMU engaged, as it helps manage engine power and propeller settings to facilitate recovery.

Other maneuvers might have risks associated with them, but they are not outright prohibited under these conditions. For instance, while stalls and high-speed turns may pose challenges, they do not specifically require the PMU to be operational for safety, as the pilot can still manage control inputs. Similarly, cross-controlled snaps, which involve specific control surface movements, may be executed but carry their own set of risks without the PMU. Thus, the emphasis on spins being prohibited reflects the heightened risk of an uncontrolled situation without the assistance of the PMU.