What does Bohr mean by "applied at least twice"?

Question

In an interview one day before his death, Niels Bohr said that "the spacetime or energy-momentum relations must be applied at least twice otherwise they are not defined."

If we send an electron through a diaphragm with a slit, the location where it hits the screen can be predicted by measuring the momentum transferred to the diaphragm. However, If the diaphragm is fixed to a support, we lose the ability to predict the location of the collision with the screen.

In one of his papers, he writes that

"In an arrangement suited to measure the momentum of the diaphragm, it is further clear that even if we have measured this momentum before the passage of the particle, we are after this passage still left with a free choice whether we wish to know the momentum of the particle or its initial position relative to the rest of the apparatus. If the diaphragm is sufficiently heavy in comparison with the particle, we may even arrange the procedure of measurements in such a way that the diaphragm after the first determination of its momentum will remain at rest in some unknown position relative the rest of the apparatus. If we choose to measure the momentum of the particle, we lose through the uncontrollable displacement inevitable in such a measurement any possibility of deducing from the behavior of the particle the position of the diaphragm relative to the rest of the apparatus, and have no basis whatever for predictions regarding the location of the collision."

Does anyone understand this argument?