its a tool to set timing
B=connect b with battery +-
c=connect with 1st cylinder
A=set 10 digri from regulator
now put a point on engine wheel, loss the distributor nut, now start the car put gun on wheel, press the gun button, light start flashing, n your car timing is set, now tight the distributor nut.
remover gun from car, n thats all
for more check video on you-tube or Google it