The “stealing” of energy from tachyons is pretty much the same process as the tachyonic fields rolling down from the hill, signaling the instability.

Tachyons’ energy cannot be kept positive in a Lorentz-invariant way – whether it’s positive or not depends on the reference frame. So the fastest way to steal energy is to give tachyons a huge negative energy. Because it can be much greater than the absolute value of the tachyon rest mass, tachyonic particle-antiparticle pairs can be spontaneously created from the vacuum (with momenta which are spacelike and opposite; there is no separation between the set of positive-energy and negative-energy momenta for them).

A (multiple pair) creation of tachyons is a microscopic description of the process of rolling down the hill. In a fock basis, the coherent states moving the tachyon field away from the unstable maximum correspond to many particle excitations.

Only theories which contain at least metastable minima of the potential, aside from the unstable maxima, can support a world where physicists have enough time to make experiments.

Scalar fields in spontaneous symmetry breaking are also examples of the condensation and vacuum instability.