From the l298 data sheet the enables should be high (which is likely what the jumper does) to enable the motor drivers. Direction and braking / free running of the motor are controlled by two input pins for each motor. At a guess the VSS jumper is intended to allow you to run lower than 5V on the motor (supplying 5V to the chip logic in some other way) or may switch from powering Vss from USB or the motor supply voltage. You might have a look at
http://www.instructables.com/id/Arduino-Modules-L298N-Dual-H-Bridge-Motor-Controll/
which shows how to use a dual channel version of this with an Arduino (found with a google search for l298n) as it should describe the basic operation for half your board (and the other half will likely work the same for the second motor). I don't know what the switch does, as I said earlier you may have to trace the wiring on the board and read the data sheets to figure out what the connections are supposed to do (in this case what the switch connects to, and therefore what it does.)