With the handlebars under the seat your arms hang comfortably by your side. HP use a separate pivot point for the handlebars, like an extra headset. Advantages; comfortable over long distances, short cable runs. Disadvantages, more wind resistance, harder to manouevre bike when pushing, extra pivots in system.

With these bars your elbows are bent at approximately 45 degrees, bringing your hands closer to your chest. Advantages; aerodynamic and very comfortable, no need to adjust for different riders. Disadvantages; steering feels sensitive at first, you soon get used to it.