The common terminology used for chain adjustment is really incorrect, as the chain is never really "tensioned". In fact, if it were tensioned, then the adjustment would be incorrect. The idea is to have as little slack as possible, while having nominally zero chain tension. There will always be a small amount of tension in the chain due to gravity, which increases as the chain slack is reduced and the chain pulls itself straighter. However, this tension is very small in comparison with the tension resulting from the rider applying force to the pedals.