Budd, I see jin more as the raw material to be exploited by aiki. "Peng" is the core, "main" jin. In "making aiki," there are things that you are doing which are separate, or an addition, to the processes being done to make houkei/peng (thanks Chris!
). There are myriad variations and manipulations to keep shifting the balance of In and Yo. For example, different degrees of engagement in the meimon and tandan, and the use of the kwas (gotta find the Japanese term for these) to create the power of opposing forces via the ground, for winding force. Aiki itself includes sensitivity toward what your own body is doing, and the ability to feel the opponent's center to control his body in any chosen direction and a variety of chosen effects. It is the exploitation and manipulation of the power you generate.
But starting at Square One, I believe that describing houkei (peng) and aiki-age are the simplest way to define IP and Aiki to someone with no prior physical exposure.
P.S. How about the term for "kwa," Chris...?
Well, Cady, I'd say that what you're again describing are the application and skill methods in how to apply jin to as you say, "make aiki". The skill and conditioning is the work you do to train it and then depending on the types of pressure testing you do will somewhat define what's considered the "correct" method for applying jin as "aiki". The latter part is especially where I think many "takes" on internal strength will diverge.
To take your example of sensitivity or listening, the Chinese term "ting jin" equates to listening to your opponent's energy/power. It's an application of jin and somewhat reinforces that there are many jins but one jin. Without the table stakes of solid peng, I typically rule out anyone's credibility to speak or demonstrate much else.