He simply asserts that it happens this way, without argument and without proof. He simply repeats that this is a scientific fact, as if saying it makes it so. The womb turns out to be a terrible place. Mama runs lightly and blithely into a table and baby gets its head stoved in. Mama has constipation and baby, in the anxious effort, gets squashed.
Papa becomes passionate and baby has the sensation of being put into a running washing machine.
Mama gets hysterical, baby gets an engram. Papa hits Mama, baby gets an engram. Junior bounces on Mama's lap, baby gets an engram. Those received as an embryo are intensely aberrative. But you must take L. Ron Hubbard's word for it.