By strength most people mean using flexed muscles, especially the big muscles in our arms, shoulders and backs. But if you flex those muscles, they tense and you can't do anything more with them and have to reset them to use them again. Not only that, but flexing your muscles makes you stiff and rigid and easy to topple.

Even if you don't use "IP IS" as is often discussed here, you will be much more fluid in motion if you don't tense up, but use balanced and centered movement and momentum to move your body and aikido technique. Just relaxing doesn't make power but it is one of the first steps for making power.