Chapter 6, “Relaxation of Muscles”, in “An Actor Prepares”. The book
does not appear to be readily available on-line, though it’s in Apple
Books, and probably elsewhere.
> Dorothy L. Sayers's _Busman's Honeymoon_, before it was a novel,
> was a staged play. At its climax, Lord Peter has reconstructed a
> booby-trap by means of which a murder has been committed,
> whereupon the murderer (who has been outside) enters and triggers
> the booby-trap, which does him no harm (the intended victim was
> taller than he), but scares him into confessing. Sayers's notes
> for the production instruct the stage manager *and the actor
> playing the murderer* to personally inspect the device
> immediately before the curtain rises on the scene.
I know it well. But this is a different case. At Talboys, the trap is
prepared so that the victim is right under the point of suspension, so
that it will smash right into his head while the pendulum is moving at
maximum speed. In this imaginary case, the “victim" is standing at the
near end of the swing, where, asssuming that the suspension is reliable,
and that the “release” involves no pushing, we can know that, on the
return swing, the pendulum cannot return beyond its original release point.