Jump as high as you can, pulling your knees up toward your chest, almost hugging them briefly in the air. Land softly and immediately go again. The explosive part is the takeoff; the tuck is just proof that you got high. What you should feel is your legs driving hard into the ground and your glutes and quads firing. Land with control so you can reset and repeat without bouncing out of position. If you're jumping high and landing hard without a moment to reset, slow it down. Explosiveness without control is just chaos.