Just the revs he can get on his big legbreak and utilising any breeze helps but also as I am keeping wicket at his training i can really pick it up clearly. I know his bowling back to front after countless deliveries i can tell how much it is going to turn by the drift and no drift usually heralds his topspinner.
My bowlings massively improved over this year and I've noticed exactly as you've said - if you see the drift, then expect something a bit special off the pitch, so as SLA and you've said it must be that the ball has far more side spin. Another thing I noticed tonight was that if when you bowl and you get up on the toes in the pivot, you get more bounce and turn, so then if you want to bowl something does a lot less in the case that the bats playing for the spin I found all I had to do was bowl with exactly the same action except that you don't get right up on the toes, for me tonight this mean the ball more or less went straight on.
I have a couple of theories about drift, spin angle and turn I will share with you.
The first one being that I believe that a little topspin mixed in with the sidespin helps the ball grip and actually makes it turn more, particularly so on a hard surface (to get technical: this is because the contact time between ball and pitch is increased because relative velocity between the surface of the ball and the ground is decreased due to the spin).
Therefore the biggest turning delivery will not necessarily have 100% sidespin - it should have a little topspin mixed in too to help the ball grip. What is the ideal angle for maximum turn? That depends on the pitch. The harder and quicker the pitch, the more topspin you need to mix in.
Conversely, a little backspin mixed in with sidespin prevents good grip and reduces turn. Hence the reason the slider goes virtually straight on despite have a visible degree of sidespin on it (scrambling the seam can also help to reduce grip). The harder the pitch the straighter it skids. On some really soft pitches, sliders can turn just as much as regular leg breaks.
Now bring in drift into the equation, which is purely a function of sidespin. The more sidespin, the more drift. So we have these options:
Topspin: Just Dip.
Topspin + Sidespin = drift, dip and big spin.
Pure side spin: Drift and big spin.
Backspin + Sidespin = drift, skid, and minimal spin.
Backspin: Just skid.
This is why the slider is so much more effective than a straight backspinner- because it has an element of sidespin, it still drifts like a legspinner, it just doesn't turn much but rather skids on.
(As you can tell - I'm a big fan of the slider)