Line Lock just locks the front tires and not the rears.
Line Lock is a solenoid that is used inline on your front brakes. You flip a switch and hold the brakes and line lock will keep that amount of pressure on the front brakes and none on the rear, until you flip the switch back off. That allows people with manual transmissions to pull up, flip the line lock switch, hit the brakes, then use the clutch and gas pedal at the same time to do a burnout, without having to worry about using three pedals with two feet.
If you have an automatic and can powerbrake in your car, then you do not need line lock.
99 Chevrolet Camaro Z28 M6 - 6.676 @ 103 in 1/8, 10.512 @ 130.2 on street tires, H/C/I
93 Eagle Talon TSI AWD 5spd - Built 6 Bolt, 16G Evo3, HKS 272 Cams - under construction
94 Mustang GT - 5.0 bored and stroked to 347, Vortech V1 @ 10 PSI, Tremec TKO 600 - PIA