This is a very good thread. This subject was treated in many various posts before... but never in a complete way i feel.
According to what i know:
RUBBERS
LP with good friction allow to change the spin on the ball. Of course more friction = more variation
The most important factor is the sponge: + sponge = + variation. Moreover soft sponge should be better than hard sponge in this.
A flex/soft blade can help a little too
TECNIQUE
I think in order to return back vs back with lp you need the same stroke you use to give the no spin, but if you do it well (and with the proper pimple) you not only succeed in taking off the spin but also in reversing it in the opposite direction.
1) The worst (imho): fast brush under the ball, grasping it with the top of the pips. ABSOLUTELY this works only vs good backspin and i found it to be very inconsistent.
2) The better (the one i think the pro use): brush the back of the ball pushing foreward. This stroke allows to use the sponge and its able to handle (wiith good sensibility) backspins and no spin (against no spin it's harder to create good backspin). In fact is a very "choppy" push. If you look at joo for example you will understand what i mean, just look at how ho goes downwards when he pushes. It is not an easy stroke because you have to read the spin well and brush heavily to obtain the spin, and if you dont the ball goes out because such lps are not slow... but it's also very effective when you can use it because it allows you to change between back and no spin, difficult to read for the opponent. By the way, i think this is also the proper tecnique to push with inverted... with lp you have just to create more friction of your own going more downwards.
Around 0:40 [youtube]https://www.youtube.com/watch?v=zL0ApAwAidM[/youtube]
(Also i think with lp in EVERY shot, chop for example, you should almost never just brush the ball, but ALWAYS go foreward to use the rubber... and in general it's true also for inverted i guess...)
It's a so fundamental stroke for a defender... so important that, for example, Leatherback wrote in his blog about the period in china that he knew a trainer who had met joo se hyuk who had explained him he used 1.5/1.7 sponge not because it was better at chopping, but to be able to push backspin (and also attacking...) better at the table!!!
I found this stroke to be pretty good with FII, FIII, P4 and F soft, all in 1.0 mm, but not really easy to control. Of course for every rubber the tecnique is the same, but you have to adjust the angle and the proportion of the foeward and downward movement!
Now with my p1 0.5 i have more control (because it's slower), but the variation is more difficult, i pretty much give no spin all the time and twiddle to create backspin. I only reach some good backspin against a very backspin ball, but very rarely... but at my level it's not such a problem and in some circustances can even be a good thing... most important is to keep the ball low and fast. I plan to try p4 0.5 mm in the future (possibly on a Victas matsushita), because i think would really help the variation and the quality of the pushgame... but of course i don't know if it suits the rest my game well...
Sorry for the long post and the bad english. I hope many can partecipate in the discussion!