The theory is that pen is too permanent, too final, and the automatic response is, therefore, to try to complete something.
Using a pencil should remove that pressure.
Conversely, I have often felt pencil to be too impermanent, transient, subject to wear and tear, and accidental erasure.
But I suppose it depends on what you're doing. If the purpose is not to create something final then the fear of transience should be unwarranted.
It's a case of retraining the mind, to break patterns, approaches and workflows.
I think the very nature of a pencil itself is symbolic: it is sharpened ready for action but wears down until it becomes unusable.
We must then re-sharpen it, and with each cycle the pencil gradually disappears; the passage to an end comprising multiple mini endings.
The trick, however, is not to be afraid of them, but to embrace them. For they are markers of progress not loss.