What is the difference between Tenergy 1.7 1.9 2.1?
Butterfly Tenergy is not that different to any other rubber when it comes to differences regarding the thickness you use. In a nutshell:
Butterfly Tenergy 2.1 is the fastest, spinniest and most control for a looping.
Tenergy 1.9 is in between
Tenergy 1.7 is the slowest, least spinny and has the most control for less agressive play.
You can loop with any thickness, it depends on your blade too. The thinner you go though, expect that you need more precision when you go for a big swing because you have less rubber to work with.
Tenergy 1.7 is quicker but not more powerful. You have less catapult but the ball hits the blade quicker and leaves quicker too and encourages a more drive style which can be very fast.
If you are a serious looper and want to experience the true Butterfly ratings that they advertise make sure you use the 2.1 as most professionals who use Tenergy use 2.1, and Butterfly most probably does it's test primarily at maximum thickness.
I will write more soon on this topic, but must run off to play now, sorry.