The ‘critical moment’ refers to the moment when proceeding to perform a movement whilst consciously continuing to give the new directions, i.e. when gaining a familiar end by new means-whereby. It is an almost universal difficulty to keep the directions for the new use going when engaging in a familiar activity, and Alexander solved the problem by subjecting the activity to be engaged in to a fresh decision:
According to this procedure the subject starts by consciously projecting the directions for the means whereby he will gain a certain end, and, at the critical moment of going on to gain this end, makes a fresh decision as to whether he will employ these ‘means whereby’ to gain the original end or some other.
Alexander only refers to the ‘critical moment’ three times in recounting his story of developing the Technique in UoS. The phrasing does not occur elsewhere in his writings, but there are discussions elsewhere on the strong influence of habit and its interference with a new conscious directed use.
It is connected with the phrasing ‘sticking to a decision against the habit of a lifetime’.
‘Psychophysical unity – and the critical moment’ by Robert MacLeod.
‘Let’s get rid of “group teaching”!’ by Don Weed contains a section on ‘the critical moment’.
 The Use of the Self by F. Matthias Alexander (Methuen, 1932), p. 41 fn1.
 ‘Psychophysical unity – and the critical moment’ by Robert MacLeod in Direction vol. 1, no. 1 edited by Jeremy Chance (Fyncot Pty Ltd., 1990), pp. 15–17.
 ‘Let’s get rid of “group teaching”!’ by Don Weed in The Congress Papers: A Spirit of Learning Together, edited by in Jeremy Chance (Direction, 1992), pp. 150–68.