Depends on where the horse is at in its training... but in an established horse it is: rattle + bump the inside leg at the girth.

Using a tool like a 'tap metronome' is a great way to quantify your stride horses tempo.

The FEI needs to define this before they can start persecuting their people...

I know I have changed my tune over the years: I wish 'on the vertical' was abolished.