Dynamical plasma compression in pinches accelerates ions by interaction with the driving magnetic piston. Once the ion collisional mean free path exceeds the pinch diameter an ion runaway is forced by inhomogeneous electric fields and multiple reflections at the magnetic piston. This combined gyroreflection-acceleration preferably taking place in high current, high performance devices is very efficient towards the end of compression since the kinetic ion energy increases rapidly at small pinch radii leading to fast (beam) deuterons (typically 100 keV) that undergo fusion reactions with slow (target) ones of the thermal pinch plasma. Experimental evidence is given by reaction proton spectra taken at the high voltage plasma focus SPEED 2.