Piggy backing off of this followup, bond prices move inversely with yields. Ceteris paribus, a -0.5% yielding bond commands a higher price than a -0.1% yielding bond. Bonds are quoted in yields but traded on a dollar price (typically as a percentage of par) hence the capital gain I implied above being realized with the higher price should yields/rates fall further.
So for example let's say you bought a -0.1% yield, but newly issued bonds are now only offering a -0.5% yield you can now actually sell your bonds paying out -0.1% at a profit, since even if they paid you a 3.5% premium on the price you paid, it would still be more attractive than the rate on offer for new bonds.