This rule refers to the grip power of your dominant and nondominant hand when holding a pistol.
It’s a rule that’s hard to apply in practice, as the amount of pressure exerted by your hand is difficult to estimate. See for yourself how hard this is. Test it out on your buddy. Ask him to first squeeze your hand with 60 percent of the full amount of pressure he is capable of generating. Then have him repeat the squeeze, but this time at 40 percent of full pressure. What you’ll discover is that this is far from an exact science.