Split Square Thru is a variation on Square Thru in which only two of the dancers start the call. They do a right pull-by and then 1/4 In as usual, to face the other two dancers who must wait for them to do this. Then everybody continues with the rest of the call. The ones who start do the indicated number of pull-bys, while the others do one fewer.
For example, in a Split Square Thru 3, the dancers who are initially facing do three pull-bys, while the others do only two pull-bys.
Note also that those who don't do the first pull-by will be doing their first pull-by with their left hands.