Let us suppose we confine an electron in a volume v the we can know the uncertainity in its velocity. Now suppose we take a small volume d(v) and observe it . There are only two possibilities we find the electron or we do not find it . If we do not find it then we haven't interacted with the electron in any way but we have successfully reduced the confinement of electron to v - d(v) still keeping the uncertainity in velocity same thereby violating uncertainity principle . Thr key idea is measuring the positions the electron does not occupy and in a system there could be probability that the electron be absent from a space d(v). We are extracting information about electrons from thier absence rather than their presence.