[cVwEq]

One way to make money is if you sell the bond at a higher price later to another buyer. From the article:

“Why are people buying at negative yields? It is mainly in expectation that you’re going to be able to sell to someone at a higher price later on,” said Andrea Iannelli, investment director, fixed income at Fidelity International. “Whatever the yield you have to assume you’re going to make more on the capital gain than lose on the yield.”

So Y < X, but if you bet you can sell at price Z to another buyer later, Z > X and you profit.

As an analogy I just thought up: it's kind of like overpaying for a house, thinking that in time the house value will appreciate.

[madcaptenor]

Or just paying for a house, thinking it will appreciate. (The "yield" of a house is negative, because it costs money to keep the thing in the same condition you bought it in, as anyone who owns a house knows.)