  We study gauge-invariant approximations to the Yang-Mills vacuum wave
functional in which asymptotic freedom and a detailed description of the
infrared dynamics are encoded through squeezed core states. After variationally
optimizing these trial functionals, dimensional transmutation, gluon
condensation and a dynamical mass gap of the expected magnitude emerge
transparently. The dispersion properties of the soft gauge modes are modified
by higher-gradient interactions and suggest a negative differential color
resistance of the Yang-Mills vacuum. Casting the soft-mode dynamics into the
form of an effective action for gauge-invariant collective fields, furthermore,
allows to identify novel infrared degrees of freedom. The latter are
gauge-invariant saddle-point fields which summarize dominant and universal
contributions from various gauge-field orbits to all amplitudes. Their analysis
provides new insights into how the vacuum gluon fields generate gauge-invariant
excitations. Examples include a dynamical size stabilization mechanism for
instantons and merons, a gauge-invariant representation of their effects as
well as a new physical interpretation for Faddeev-Niemi knots.
