no pitfalls really (since this is basically establishing the foundation for deeper things and is entraining a path of least resistance for additional energy) but I'll toss in some extra guidelines...
-be gentle, esp with the motion of the perineum - the addition of this was a well guarded secret for generations because of the martial power it adds. its like trimming the motor on a boat. a lift as opposed to a contraction.
-breaths goes not so high as to perturb the heart, not so low as to reach or go below the genitals.
-utilize the middle ~70% range - as you get more air in the lungs there's a point where more air requires more energy from the lungs for expansion. (great tip from BK Frantzis' opening the energy gates book)
-focus on the 3 main fundamental breath structures: diaphragm, psoas, perineum. those are the 3 that create the internal dynamics. so when you inhale, imagine dropping the junction of the diaphragm & psoas. this "roots" the motion of the breath and allows you to begin to stop using the sinuses and air passageways to facilitate the movement of air - eventually you get to where its just diaphragm-psoas-perineum, that's where "the breath externally disappears" and you've dropped the movement of air below the threshold of turbulence, you can no longer feel yourself breathe, but still have the "one breath" rolling motion going on. that stuff happens around the minute mark. the more "longevity" your breath gets, the more you wind up streamlining the usage of everything.
(I developed an air passageway identification exercise that may be of help - basically investigate each and every place air touches on its path and see how you may effect the local air pressure there. pull air at the tip of the nose, try the crown of the nose, maxillary sinuses, ethmoid sinuses, frontal sinuses...be gentle with the sphenoid since it is right on the opposite side of the midbrain, it'll give you a headache quickly if you use it too hard...back of throat, larynx, trachea, bronchi, upper lungs, lower lungs, diaphragm. so from a position of action, sequentially analyzing...derive inaction, completely let go of the entire air path and do not use it whatsoever to facilitate breath. only then will the breath disappear!)
I believe the look down the nose thing was partially a method to help fixing the spirit at the seat of awareness.
when the spirit is well affixed at the seat of awareness, that is partly arresting the energy potential available to the senses - so you can liken the 5 thieves to the physical component of 12 thieves, the cranial nerves. air flow across the olfactory nerve stimulates it, neural firings beget neural firings. so relaxation is the method for some of them, like the trigeminal; coherent breath is the method for others, olfactory, vagus; stillness, vestibulochlear, and so forth. there is a matrix of feedbacks to mitigate as you calm your body into lower consumption states - so combined with harmonious breath, we have a relative production maxima at a consumption minima. sure, the production maxima may be higher, but in those other situations the consumption will also be higher still - so think net positive balance.
-posture helps flow. if your spine isnt decently straight, the diaphragm motion does not flow smoothly, like if you are hunched forward.
once you figure out how to keep your spirit at its seat, it shouldnt require a lot of willpower to do so. it becomes harder if you entertain thoughts - an old master once said, leave the front door open, the back door open, but do not invite them to stay for tea! whenever you catch your mind wandering, simply return to the breath, reestablish the breath protocol. remember this energy is light, so even where there is higher flux density, it is still light. mind is gravity, Yi directs Qi, it will stagnate if you let it bog you down but it will also soar if you let it /\