A student really starts to progress in Ki development when he or she makes the leap of relying on the principles rather than the muscle and mind power that most people rely on.
That's an awesome summation of ki development training IMO. Much (by no means all though) of what is discussed in terms of IS/IP is about training to get to this point. The common assertion to 'let the floor take the push test' or words to that effect is what Mary is saying as I understand it. The author Will Reed described it as (I don't have his book near me so I'm paraphrasing from memory) "Do this thing quickly in your mind and then relax completely knowing that it's done".
When I try to explain this to my students I often tell them about my first experience of tameshigiri practise. I was trying too hard to cut the tatami, only once I relaxed completely and trusted the sword to cut did it actually cut. In the same way I tell students to trust the aikido waza and let them do the work, trust the principles of ki development I've taught then and the visualisations and explanations I've given them to understand them and they'll work. Once you get to that point ithe fun really starts