Thanks guzzio,
but with mechanic/electric background, a bit interest, some patience and a lot of time it’s relatively easy to collect certain knowledge 
1a)
- Loosen big groove nut (hook key required!)
- Loosen the kickstand bolt 274
- Rotate eccentric bottom bracket to increase/decrease belt tension
- Tighten kickstand bolt
- Tighten groove nut
- Check tension finally again
1b) That’s also the opinion of Mr. Mark Sanders, inventor of Strida.
Read on here: Expert advice on belt tension on alloy chainwheel
Please note that the Marks.pdf link, provided by Ming cycle, doesn’t work.
If anybody is interested to read the document please pm me.
Tips from Mark.pdf torque warning!
- Correct as far I know - no grease at all.
Mr. Sanders mentioned - just in case of squeeking - common candle wax, but to be used only at the sides of the belt and not the teeth.