Originally Posted by
HandsomeRyan
Because the the spring in the tensioner is firm enough to hold tension on the lower part of the chain when there isn't much pressure on it. While pedaling forward the top of the chain is under pressure but the bottom part of the chain is not. The first time you stop pedaling or apply back pressure this will reverse though and you'll have a moment of play where the pedals stop but the wheel keeps moving with it takes out the slack and then it will jerk to a stop.
This. Think about it: when you pedal forward (apply pressure), the top line of chain goes perfectly straight and is tensioned quite a bit but the bottom line can be slack. When you pedal backward, the top line of chain goes slack, and the bottom line gets tensioned. The chain tensioner would have to be
very strong to resist the chain wanting to go perfectly straight on the bottom. This is why Sheldon says that even chain tensioners for downhill MTBs aren't strong enough for fixed gear use.