The Sweet Spot is a reference to hitting a tennis ball in just the right spot off a racquet. It’s not the ONLY way to hit the ball back over the net, but it’s the way that sends it back the fastest, with the most control, least amount of effort, and it doesn’t put any strain on you. If you miss the sweet spot, you have to work harder to get the same results, and you’ll be putting up with stress and strains on every shot.
It’s an apt metaphor.
Heavy Lifts vs Slow Burns - heavy lifts are NOT in the sweet spot