[Nonfractal]

As a kid , I used thumb "plunger" and finger "trigger" scalextric controls. 
When I came back to slot raacing about five years ago, the controls in use at the club were (mainly)  DS trigger controls. 

As part of my job I attended RSI "repetitive strain injury" training. 
Many on the course (including the trainer) had already gone through surgery to repair carpal tunnel damage. 

The cause? Incorrect extended use of keyboard, mouse and mobile phone (texting). 
The trainer's biggest bug bear was teenagers using their thumbs to mobile text.
"An accident happening in slow motion" 

Her big message was that your thumbs are precious , don't overuse and abuse  them. 

Having said that, unless you are racing as a full time job or at 24 hour events, other aspects of your life are more likely to cause RSI than slotting. 

I use my middle finger because:
- it gives me better fine-motor control 
- the contact-free light spring trigger on the slot it scp2 seems to respond better 
- the controller can be stabilised by the outstretched index finger, avoiding the need for the 2  handed grip to keep the  controller correctly orientated and making blood flow to the trigger hand easier for your cardio vascular system.

I'm a "left thumb in my jeans pocket, controller hand hanging loose by my side " kind of racer. 
(Unless its digital oxygen racing where the 2 handed grip really is the only option) 

Hope this helps 

Alan W