Notice that there are two center wheels in the movement – the upper wheel connects to the split-seconds hand, while the lower wheel connects to the primary chronograph seconds hand. The split-seconds functionality is essentially achieved by a set of levers that form the jaws of the clamp we mentioned earlier. The upper wheel for the split hand is mechanically connected to the lower by a spring-loaded lever with a ruby roller on the upper wheel, that rides in the low spot of a heart piece cam (exactly like a normal chronograph return-to-zero heart piece cam) on the lower wheel.

Pushing the split button causes the jaws to close around the wheel carrying the split-seconds hand, while still allowing the lower wheel to continue turning, and hence still keep recording time. The roller and lever on the upper wheel stop moving, but continue to press against the cam on the lower wheel (which continues to turn) under the influence of a tiny spring. The split time can now be recorded. When re-activated, the jaws of the clamp open, and the upper wheel is free to rotate back into position as the ruby roller finds the low point in the cam, under the influence of the tiny spring we just mentioned. The two hands are now superimposed again.

The split-seconds mechanism is not considered one of the "high" complications because of its complexity (it's one of the three complications traditionally found in a "grand complication," along with a perpetual calendar and minute repeater). Rather, it gets its status from the fact that for it to work, everything has to be adjust extremely precisely, requiring a lot of skill from the watchmaker.

The video below demonstrates how the split-seconds mechanism works, concurrently showing both the dial and movement. Watch carefully when the split button is pushed – you'll be able to see the jaws of the clamp closing on the upper split hand wheel, while the heart piece cam on the lower continues to rotate.