Not sure how IS/IP is going to help in this case. Usually when the strength bogey man comes up it really is code for fluidity and a concentration on what might happen rather than the technique itself. It is also - I have to say - another one of those terrible words like "relax". Lot's of meaning but not particularly helpful.
Just want to touch on the IS/IP/aiki relax part. Thought maybe I can help clear that up a little ... or muddy the waters some more.
I know we've all heard "relax" and variations of it through our aikido career. Usually with years of training, it eventually starts taking shape, but there's not really any training methodology to improve "relax".
With IP/IS/aiki training, "relax" is integral to the training exercises. At least in the beginning, it was for me. Let me explain.
Starting with a basic push exercise. Stand in a natural feet side by side, shoulder width apart stance. Arms outward at 90 degrees to the body, palm outward, fingers pointing upwards. Elbows don't have to be locked for this. Now, have "uke" push on your outstretched right palm. Start with an easy push and have uke add more strength.
1. This is the tough part. Imagine that you're palm is on the wall several feet away. Not just think it's there. Try to get the feeling of your palm pushing on the wall. In the beginning, you'll extend your arm physically to get this started. Once you have that, keep that. On top of doing that, imagine that you're bringing that wall into your spine. You have outgoing and incoming energy at the same time. Have to have both.
1. Let all that energy/strength coming from uke go through your arm, down your spine, down your *left* leg, and into the ground. Right palm to left foot.
2. Start with light pushes and as you get more force coming in from uke, go to your fail point where your shoulders tighten up, chest muscles up, your upper body breaks and you bend over. Get to the very beginning of that point and tell uke to hold his push there. Work through that until your structure is back to being good, you have contradictory forces again, and the right palm-left foot path is clear. Then have uke add more force.
These kinds of exercises force you to relax in order for them to actually work. If all you're doing is muscling things, uke will push you over easily. You'll topple from the top.
If you don't have contradictory forces going (There are more. For example, spine up/down, but if you haven't done these, then getting one set working at the same time is tough enough) then you can only go so far with that basic cross the body path. Remember, this is kindergarten level exercises.
This isn't anywhere near a complete exercise.
You can add in the other contradictory forces as you get better. You can have uke push harder and stronger as you get better. Throughout these kinds of exercises, you learn to relax. You have to, or else the exercises fail. While you're concentrating on contradictory forces and cross the body paths, you have to relax to get everything to work. Part and parcel of the exercises.
Now, just as an added benefit. Wondering what in the world this has to do with aikido techniques? Picture this sequence of exercises:
1. Exercise as above.
2. Modify the above it so that your fingers are pointing straight out at the wall. Uke uses two hands and grabs your right wrist to push.
3. Modify the above so that your hands are now straight in front of you, pointing the way you are looking. Uke uses only one hand to grab your wrist and push. Oh, wait, this is the standard wrist grab in aikido techniques. Hopefully by now, there's no more fighting uke's grab, tensing of the shoulders at uke's grab, etc. Body is more relaxed. Etc.
In 5 years, I learned to properly relax from IP/IS/aiki exercises than I did from 15 years of being told to relax in Modern Aikido. Not that I'm at any significant level of relaxed, coordinated, no-slack, structured body, but I'm way beyond what I was at the beginning of training IP/IS/aiki.
Hope that explanation and example exercise helps understand how the IP/IS/aiki exercises give a fundamental added benefit to Modern Aikido training.