You wear a strap around your chest just under your bra line. The front is a plastic (only about 10cm long) and the rest is extendable elastic. It is actually pretty comfy. The plastic bit has sensors on it which picks up your heart beat. You wear a watch that, when turned on to the correct setting, reads your pulse. Really quiet simple. And it is very accurate. The equip at the gym I go to also picks up my HR when I wear my belt thing... Soooo, no weighing down of pants

HTH

MG