I think that's where I get lost too like a sumi otosh(sp?) ura. If I breath in during the tenkan motion, then out during the cutting of the hand, what's left for when I step behind to actually drop them?
You should practice exhaling longer than you inhale. Start by sitting and inhale slowly for a count of two and then try to exhale for a count of three. Practice this whenever you are sitting and when you can do this easily increase the exhale count to four. Make sure your counting is done evenly for inhaling and exhaling. Over time you can increase the exhales.
While sitting still I can do a four count inhale then a twenty count exhale.
After you get proficient at exhaling for a longer time than inhaling , try it while walking and then try tailoring your breathing count to match the count or rhythm of the technique.
I can get halfway through a 16 count bokken kata with a one count inhale before I start the kata and an eight count exhale. Then I take a quick inhale and finish the second half of the kata with an eight count exhale.