Tohei goes on for a couple paragraphs about joining the army and not being able to do this breathing method while marching. So, he came up with a new breathing method.
Haven't got the book to hand but we practice this breathing method here. If you're interested in these things then I would definitely recommend finding out about O Sensei's misogi exercise called 'the rites of spring' as preserved by Hikitsuchi Sensei. Many of O Sensei's breathing/kokyu exercises are contained within it along with his kotodama practice. Notably it contains the rowing exercise which he appeared to consider fundamental to his aikido. This exercise is one of those taught in the ki society from the early stages of training. As it is in Iwama Ryu aikido and elsewhere (notably Hikitsuchi of course), so two 10th Dans and O Sensei's longest continuous student (Saito) definitely seemed to think it vital to aikido practice.
Interestingly where we train we tend to practice some of these exercises deliberately after a period of hard aerobic training where we're out of breath, it's extremely hard to do but very worthwhile. I've also noticed that at kendo keiko my sensei often makes us do lots of fast okuriashi practice followed by men cuts with loud kiai so that we can 'learn to focus on our breathing because it helps learn ki ken tai ichi' in other words, basically the same exercise in terms of aerobic changing to deep controlled breathing in a short space of time.