Clementine, the split back cut or quarter cut (as Eric calls it) is one of the safest of all tree falling techniques, it was how we were trained for commercial manual harvesting of complex and hazardous trees and is the technique we use all the time (because of the increased control it gives you).
What Eric is doing is only cutting half of the back cut, then driving a wedge into that cut until it is tight, then he moves (around the back) to the other half and completes the backcut up to the thickness of hinge wood he has already decided is required for that particular tree.
Since the wedge is lifting the tree, there comes a point when completing the back cut (on trees with no back lean) when the tree is just tittering on its point of balance...with the tthree palms each with a wedge and completed back cuts the connection throught the interwoven fronds allows Eric to fall all three.....the only serious risk to Eric comes from the lack of desirable escape route due to the fence being so close to the stems.
As with all tree falling techniques this should not be attempted by untrained persons, proper assessment of the suitability and safety of any individual tree to be felled can only be completed by experienced trained persons, with the appropriate equipment and PPE