The size of the fretboard makes it easier for us to navigate and make use of different techniques which may be considerably more difficult with other music instruments. The fretboard is rather thin (considering that the Cuatro only has 4 strings) and therefore we are able to have a greater reach in terms of marking frets at a distance and in this case with different fingers not typically used.
In the demonstration video below, the thumb is being made use of in order to play the A string and assisting the musician to add notes to chords. It is important to learn how to make use of different ways of marking chords and essentially marking the frets because at times we may find ourselves in a position where changing between one chord and another can be complicated because of the positioning of our fingers. Conventionally, we mark chords with our fingers excluding the thumb.
In the example, the musician chooses to avoid making a bar by adding his thumb in the chord change, in order to make it easier for him to change between chords. Take a close look at the example and see how you will be able to make use of this technique. As your hands become more flexible then you will be able to make use of this technique more and more. It will save marking bars in some occasions as well as liberating the remaining fingers to play other notes.