The Dzogchen kama, or oral lineage, taught four foundations prior to the practice of trekchod and thogal.
4. Spontaneous presence.
By mastering these four foundations one arrives at naked awareness beyond mind. As your experience of the nature of mind becomes more and more subtle, you elicit the unobstructed, transparent nature of rigpa. That subtle experience of baseless clarity transitions one to the practice of the four methods of resting which are the main practice of trekchod.