Movements are never the same twice, even when you are trying to do that same thing over and over. Variability is an inescapable fact of trying to organize and run a complex system such as a human body. But there is more than one source of variability in movement; there's noise, and then there's redundancy, and these are not the same thing.
Our movement systems are redundant; specifically, they always have more degrees of freedom available than are ever required to perform a given task. This means that there is always more than one way to perform any given task, and this can range from slight variations to complete reorganisations.
Redundancy is a feature, not a bug. It means that we can reliably achieve a task goal in the face of perturbations that range from trial-to-trial fluctuations in execution up to surprises like tripping or the sudden appearance of an obstacle. However, it poses two related control problems. First, a problem of action selection: given that there are many functional organisations of degrees of freedom that could solve that task, which do we choose, and why? Second, a problem of action control: once we have our degrees of freedom organised, we still have some left over that need to be actively controlled; how do we do this, and why do we control them the way we do?
Part of the answer to these questions is the formation of a synergy. A synergy is a particular kind of way of organising system degrees of freedom, such that they work together to compensate for variability. A simple example is pressing down with two fingers (2 degrees of freedom) to produce a single total force (e.g. 20N; a 1 degree of freedom task). The system is redundant (2DOF working to control 1DOF) and so there are many ways to achieve the task (one finger could do 5N, the other 15N; or they could both do 10N, and so on). Trying to maintain a single force output is tricky though, and there will be variability. A synergy is present if, when one degree of freedom slips up, the other one automatically changes it's behavior in a way that preserves the outcome goal. When a synergy is present, therefore, most of the variability will be along the line that describes where the successful outcomes live in the space of possible outcomes. In addition, no control needs to be exerted to control this variability away, because it's just the synergy doing what it was built to do (hence this particular subset of possible outcomes is often referred to as the uncontrolled manifold).
So, which functional organisation of system degrees of freedom should we select? The one that operates as the synergy that compensates for variability in a way that preserves the task goal. If we select that synergy, it makes the control problem straight-forward (or at least, manageably low-dimensional).
There are a variety of analysis methods that work to decompose variability in the performance of a task to look for evidence of a synergy at work; if most of the variability is constrained in a way that reflects the task goal, then a synergy is active. These methods include the uncontrolled manifold analysis, optimal feedback control, nonlinear covariation analysis and tolerance-noise-covariation analysis.
I have recently been working with the uncontrolled manifold method in the context of targeted throwing, and using it to develop a detailed analysis of the task as well as trying to connect it to the perception of affordances. This work has clarified something very important for me: I really like the underlying motivation for the motor abundance methods, and I think they are all trying to meet the problem of action control head on. However, they are all, in general, completely underconstrained methods right now, because none of them come with a theory of what determines how the perception-action system identifies the synergy to enact. They can all spot one in the data if it's there, but they don't explain how it came to be. So, like dynamical systems, what we have is a good toolkit in search of a good theory, and, as usual, the solution is the ecological approach; specifically, the perception of affordances.