 To elucidate: In concrete applications, one would prepare a system in some state (not exactly known) and perform a given joint measurement \(M\) of \(Q'\) and \(P'\). If it is given that, say, \(\Delta(Q,Q')\) is very small, one can safely infer that \(Q\) has been measured with small inaccuracy, since this guarantees that the measured position distribution differs very little from what an exact position measurement would give, regardless of the state of the system. Now, one would like to be able to infer that in this case the disturbance of the momentum \(P\) from \(P'\) must be considerable for the state prepared. But the BLW only gives us: 