No timing refers to the speed controller setting for brushless systems - you may be familiar with the timing setting on a modified brushed motor, which changes the angle of the brush hood on the end bell relative to the magnets in the can; the more you advance the timing, the greater the power and rpms. In comparison to the modified motor class, we had the 27 turn stock brushed motor class where the motor timing was fixed in order to provide a comparative level of performance.
With the advent of brushless systems, the timing of the motor can now also be adjusted electronically by the speed controller enabling incremental changes during operation (active timing). With a brushless system it's possible to make higher turn motors perform as well as a low turn motor by increasing the timing and boost settings to generate massive amounts of power and rpm, but at the expense of efficiency and durability.
So in order for the racing to be even (like the old 27 turn stock class), the 17.5 no timing class requires you to run a brushless speed controller with no timing advance - there are now specific speedo products on the market that either have fixed zero timing or a stock (no timing) programme that, when activated, can be identified by a flashing LED - these are commonly known as 'blinky' speedos.
Hope that helps...
Cheers
James
Try 'Quick Silver', the Amazing Silver Can Powered Touring Car Class at HRCCC