Proprioception as in like the hot stove syndrome. If I touch a hot stove, it will generate a response. Maybe I am not technically using the word correctly, but the grab will/should in most people generate a natural response to do something, usually uke will stop moving forward, attempt to center his/her body, and pull away.
If you grab my wrist, for example, there is typically a proprioception that is communicated to me. If you pull, I will typically pull back and break contact with you. It does not make sense to me to continue to allow you to hold me. However, if you are "good" it is possible for you to make contact, not generate any "communication" at all, and leave me disoriented as to what to do and hence, it does not generate a response to pull back.
I talk to folks I am working with about giving me a reason to hold on. There are two reasons for me to hold on. First, you relay no message to me at all through your connection as to where your center might be. Two, you may communicate your center, but I realize that by letting go, there is worse things that might happen to me.
In Ikkyo though, for most beginners, grabbing is not the thing to do. They have not developed the skills necessary to avoid an incorrect response, and I submit that they will never learn it by reinforcing the bad things that go with the grab.
Therefore, I find it better to not grab at all, which eliminates this, and then requires them to learn how to control uke through their structure. The interesting thing I think is that by simply not grabbing, Uke immediately loses this proprioceptive reference point, which should by Nage more time, and causes Uke to continue through with his attack, therefore, committing his center "more".
It can be a scary thing to "let go" (no pun intended) as our fear is that by not grabbing we are gving up control. AND by not grabbing earlier, we are allowing uke more time to attack us which in our logical minds is a bad thing as conventional wisdom would say to us that grabbing often and early is much better than not grabbing and doing it later!
In reality, we find in our practice that sometimes not grabbing and allowing uke to continue through, gives us Time, causes uke to commit fully, and screws up his Decision/Act loop sense the feedback he needed to continue on to the next step was never there!