Re: Tsuki Iriminage - Step back hand grab
He is doing iriminage... not an irimi opening. Iriminage does not necessary dictate and "irimi" opening. Iriminage can be done from many openings out of tsuki. It is a little more challenging from a tenshin opening, which is what he is describing. The "step back" is called tenshin (not techin). It is step back on the diagonal allowing the nage to get off the line of the attack.
So....Make sure you are not stepping straight back. You should be on a slight diagonal.
Also, you should not be actually grabbing for uke's hand. As you step back on the diagonal and getting yourself off the line of attack, you forward hand should sort of cut down of top of uke's forearm and then slide down to his wrist.
So if you started with your left foot forward, you should now be right foot forward.
At this point you start to turn uke's wrist over using a nikkyo type grip. Ukes thumb should go down towards the ground and back up, not over and then down. When you have uke's wrist locked, you step through with the rear foot while also raising up uke's arm as if you were doing ikkyo exercise. As you step in, you can begin to release the grip, let you hand slide down back towards uke's elbow and still control uke's elbow as you execute the iriminage through.
Points to consider, watch your maai or distance. Make sure uke would actually strike you if you did not move. Make sure uke is not tracking you as you step back off the line of attack (insincere uke's will do this). When doing this tenshin opening, it is vital that you do not step back to far away from the uke. If you are reaching forward to much you are to far away. (also if you are too close you will feel bunched up in your arms.) Make sure you do not step straight back as you are still on the uke's line of attack.
Also, as you are drawing your hand down ukes arm to grab the wrist try to draw uke into your center. You will sortof be bringing ukes strike forward on off center as you are stepping back slightly on a diagonal. As this point Uke should be off balance.
Finally, your motion for all of this should start and end with your center.
And, finally, fianally, play with it some more and ask your sensei or senior student. As your partner what he feels or doesn't feel. Be quizzical to figure out how it works.