However, the story is different when I pivot on one foot and turn the other to make my partner fall. Even though he doesn't resist my technique, I still feel like I have to use enough force to make him fall. With students who are more massive, it's even harder.
I try to use spiral motion as my sensei advises the class. However, by the time my partner falls, I almost lose my balance.
Does anyone have any idea how to do pivoting correctly? How many times it takes you to do it correctly. After my 4th class, I can't still get it right.
First, disregard anything I suggest in favor of your teacher who can see you do it.
Second, be sure that your position is correct relative to his (both feet should end up in mirror position with uke).
Third, from your description, I would guess that you are shifting your hips and center slightly to the rear when you pivot in tenkan. It is easy to do without noticing it. Even when someone points it out, you will keep doing or resume doing it for quite a while anyway. While your leg should be sweeping to the rear your weight does NOT shift rear, (for the typical forward kokyu-nage throw) Rather, maintain a relatively even stance slightly weight forward, say 60/40. Do not sacrifice upright posture to do this.
Fourth, when you turn there is particular "arc of no-resistance" that your arm will track around with his arm, if ( and only if you keep good extension in the turn -- it may track high it may track low -- try not to fight for it, but to find it -- You shift forward to about 80/20 for the turn and then pivot, and resume about the 60/40 when the turn is completed, by rising
slightly on the weighted foot -- NOT by shifting to the rear foot. I know you are moving, and the weight is shifting but the RELATIVE 80/20 distribution of support should not change much, once you move in (-- its not math, its feel). Happo undo practice, whenever and where ever you can, will help you immensely in understanding how to intuitively modulate your weight distributions in motion.
Fifth, once you have turned with that correct distribution, his weight balance should come to be more forward than yours (about 80/20 on the front foot), and as soon as the turn is completed -- you move forward by essentially doing the funetori "rowing" motion forward from about the 60/40 you have maintained-- to the same 80/20 distribution that he began at. However, that sucks the remaining 20% slack out of his remaining support on the light leg, with your center about 4-6 inches further forward giving him a slight overcommitment forward in kuzushi.
Sixth, keep what you have and DON'T push off from the back foot. Many students err at this point by trying to push off the back foot to throw. That means they shift weight back and then go forward again -- (a great way to be reversed). No retreating. Not that the shift is "wrong," but it leads to very different techniques, kaiten-nage for example.
Seventh, project uke's fall further by basically "popping" the front foot (still weighted) out forward (like kicking out a prop), letting your weight drop, and simultaneous draw the rear leg in and under, to resume your 60/40 stance forward weighted, catching yourself about two inches lower, going forward. This shifts your center about 6-12 inches even further forward depending on how much you can project your center without sacrificing balance dynamically. Unless uke can support your, say 150-200 pounds, in free fall, at arms length, without a stable base of his own, while accelerating off of his last support, he will continue to project strongly into his fall.
Extra credit: It is actually important, to make the technique particularly effective, that the "pop" part, the sudden drop and catch of your weight falling, have the same rhythm as furitama or tekubi furi. It is a bio-mechanical thing, try doing for it now and work to understand it later.