 Differences between the arc directions create posts that are designed for different things, a rear arc would be better for small wheels, moving fast or going uphill while a forward direction would be best with larger wheels on a downhill or at slower speeds.Some seat posts are designed with uneven links creating what is not actually a parallelogram so that the arch approximates a straight line when compressed.Various different designs are implemented to tension the seat post, some use elastomers and others use pistons.