I think he meay be saying Sankaku as in 3 cornered irimi and Sokomen (which I think is more of a side entry)irimi?!?
I have no idea if this is right but here is my best educated guess:
The footwork for a sankaku irimi as kind of like skating where the left foot goes off at an angle, the right then comesup to meet it and almost immediately slides off to the right, each time advancing, so it appears as if you've skated past and behind uke.
Footwork for sokomen irimi is more like a funny basketball drill or something where the left foot slides forward and to the right while turning the hips, the right crosses in front of the left, and the left then moves behind uke. At that point you're behind uke.
Hope that helps