The basic idea is that the "lightning flash" simile belongs to the Long Path stage, hence its brevity and fitfulness, whereas "the leap over a deep narrow ravine" is the correct simile for the Short Path. If the seeker succeeds in reaching the other side of the ravine, he will be safely and permanently established in the truth. The ravine cannot be crossed by a series of gradual stages. If he does not succeed, then he merely stays in the darkness where he already was.
-- Notebooks Category 23: Advanced Contemplation > Chapter 5: Balancing the Paths > # 31