What is Thögal?

Recently I came across a progressive series of workshops working out of the Kusum Rangshar (by Shardza Tashi Gyaltsen) where one of the classes revolved around a practice called Thögal (ཐོད་རྒལ་). Searching for this term to determine what exactly it is has not been particularly fruitful in English-language sources other than to say that it is linked with the practice of dzogchen, so I am wondering if there is a reasonable high-level description of what it is or what it entails?