We investigate the properties of plasma fluid motion in the large-amplitude, low-frequency fluctuations of highly
Alfvénic fast solar wind. We show that protons locally conserve total kinetic energy when observed from an
effective frame of reference comoving with the fluctuations. For typical properties of the fast wind, this frame can
be reasonably identified by alpha particles which, due to their drift with respect to protons at about the Alfvén
speed along the magnetic field, do not partake in the fluid low-frequency fluctuations. Using their velocity to
transform the proton velocity into the frame of Alfvénic turbulence, we demonstrate that the resulting plasma
motion is characterized by a constant absolute value of the velocity, zero electric fields, and aligned velocity and
magnetic field vectors as expected for unidirectional Alfvénic fluctuations in equilibrium. We propose that this
constraint, via the correlation between velocity and magnetic field in Alfvénic turbulence, is the origin of the
observed constancy of the magnetic field; while the constant velocity corresponding to constant energy can only be
observed in the frame of the fluctuations, the corresponding constant total magnetic field, invariant for Galilean
transformations, remains the observational signature in the spacecraft frame of the constant total energy in the
Alfvén turbulence frame