For me IMHO, it is always finding the "circuit" from nage's hara to uke's hara once the engagement has started. I liken connection to flow of electricity except that in this case it is flow of controlling balance. The challenge is how to complete the hara to hara connection.
I like the circuit idea very much for the transfer of power, its described in the biomechanics world as the 'kinetic chain' and has nice analogies/ relevance for grounding, ground power etc.. It also can be used to highlight how it can be maximised in nage and minimised in uke