The inverse relationship between how much force a muscle produces and how fast it shortens: heavy loads move slowly, light loads move fast.
At one end is a maximal squat that grinds up at 0.2 m/s. At the other is an unloaded jump, almost no force and very high speed. Every training tool sits somewhere on that line: heavy singles, speed work at 50 percent, jumps, throws.
The curve is why a program built entirely from heavy triples produces a strong, slow athlete, and one built entirely from jumps produces a fast, weak one. Training at one point on the curve moves the whole curve a little, but it moves the nearby part most.
For a general strength client, two points is enough: one heavy lift at 85 percent or above, and one fast movement such as a med-ball throw or a jump for 3 sets of 3, done fresh before the heavy work.