Does their (your) Ukemi tend to grab, hold, or stay connected more than maybe you might see in Aikido or Judo practices? That is, riding the fall down.
With Ushiro Sensei, you literally receive no warning signs (eg. tactile, tension, etc.) before you find yourself propelled, so that you do not have an opportunity to grab or hold. With my old wrestling background, and the work at SBK and with Ushiro Sensei, I find that I have no problem remaining relaxed as I am being tossed. That allows me to be able to remain connected. For example, when most people do sacrifice throws on me, I go with it and then use the momentum to change the roll so that I end up on top. I do not get enough "info" from Ushiro Sensei to be able to do that with him.
Ushiro Sensei uses his Ki so that no matter how hard you get tossed on the ground by him, you end up not getting hurt. It teaches your body at a preconscious level to be able to receive the throws safely.