a bit of both actually. it keeps the chain taut. theres not really much "tightness" actually. I got the idea from regular chain tensioners, for cars and motorcycles.
then thing is that this kit did not like any slack at all on the tensioner. when there was more torque on the engine after a while the tensioner kinda moved back towards the front wheel and it looked like it tilted a bit towards the spokes, even though I had tightened the thing "gorilla tight".
I had thought of drilling a hole through the tensioner plate, into the chainstay of the bike, a-la norman, but I decided to look for an alternative to drilling a hole on the frame. (the frame is a raleigh and it think its nice
The bikes coasts freely with NO vibration from the chain, very smooth. I think because its the "sealed bearing tensioner" and not the regular one that I was able to get away with this. I remember trying to get the stock tensioner taut on my 2stroke kits when I first got them, and NO-GO. too much friction the tensioner wheels would not spin properly. the stock tensioners I think run better with a good amount of play on the chain. it rattles a bit but it they run just fine. I think it was actually one of YOUR posts that convinced me to let the chain loose, before that I wanted to get it to have some "tension".