Well, first off, it's tension on the spokes as a reaction to torque applied to the hub. Secondly, it's physiologically impossible to apply more force to the pedals when backpedaling than it is when pedaling forward. If anything, you can apply significally more force pedaling forward, especially if you have good clipless foot retention, and use your arms and hands on the bars to react against your legs when standing to accelerate or climb hills.