Psfred has given some good insight into the chain stretch experienced by some engines. I would add only that many vehicles experience a chain breakage or jump upon initial cold startup. This is what happened to our old 420. The reason is based in the fact that upon the previous shutdown, the engine momentarily rotated backwards, (compression, backfire, detonation, dieseling or what-have-you) leading to a jumped tooth or two. The tensioner does no good in these cases since they are on the other side of the engine travel. Of course, this is only a problem when you have excessive stretch to begin with.