The idea is that rather than anchoring to a single point at the centre of the traveller track, you anchor to two points, one off to either side of centre. This means that there is always one rope with a component of its tension that stops the car from moving, even when centered. This is not the case with a single anchor point. To look at it another way, it's fairly easy to push even a very tight rope sideways, but much harder to move it in the direction that it's pulling.

I finally got round to putting a decent split tail on my main on Tuesday, so I'll be interested to see how it holds up this weekend.

Paul