  In the past two decades, our understanding of the transition to turbulence in
shear flows with linearly stable laminar solutions has greatly improved.
Regarding the susceptibility of the laminar flow, two concepts have been
particularly useful: the edge states and the minimal seeds. In this nonlinear
picture of the transition, the basin boundary of turbulence is set by the edge
state's stable manifold and this manifold comes closest in energy to the
laminar equilibrium at the minimal seed. We begin this paper by presenting
numerical experiments in which three-dimensional perturbations are too
energetic to trigger turbulence in pipe flow but they do lead to turbulence
when their amplitude is reduced. We show that this seemingly counter-intuitive
observation is in fact consistent with the fully nonlinear description of the
transition mediated by the edge state. In order to understand the physical
mechanisms behind this process, we measure the turbulent kinetic energy
production and dissipation rates as a function of the radial coordinate. Our
main observation is that the transition to turbulence relies on the energy
amplification away from the wall, as opposed to the turbulence itself, whose
energy is predominantly produced near the wall. This observation is further
supported by the similar analyses on the minimal seeds and the edge states.
Furthermore, we show that the time-evolution of production-over-dissipation
curves provide a clear distinction between the different initial amplification
stages of the transition to turbulence from the minimal seed.
