Complex sentence: This clause does not result in a permanent increase in the number of Senate seats, however--instead, an attrition process is applied by which senators leaving office through normal means are not replaced until their province has returned to its normal number of seats.
Split into simpler sentences: This clause does not result in a permanent increase in the number of Senate seats, however. Instead, an attrition process is applied by which senators leaving office through normal means are not replaced until after their province has returned to its normal number of seats.