[RSX]

You'd need to do it like this :
OnPlayerEnterCheckpoint
Check if it's the right player/checkpoint(if they're streamed) and then
you'd start some idling process (such as being frozen with TogglePlayerControllable) and add your timer :
SetTimer( "your_public_function" , runs only once )

public your_public_function(playerid)
{
Here you'd stop your idling process (such as unfreezing player) and probably give further commands, for instance the next checkpoint.
}

If you didn't understand it yet (I really hope you did) someone or I could write short example of this.