Pressing the "lock" button with lock the shkval to the ground as oppose to swaying around with the movements of your aircraft. Just beyond the max range of the 25L the shkval will auto lock onto objects with hard edger(aka a vehicle that you have the cursor over).
Regardless of whether or not the shkval has acquired a lock on an object with hard edges(which it always will just before maximun range) it will still fire and follow the laser(cursor) once in range.
You can even hit multiple targets with one pass. example: lock a target and fire once in range, count 4 seconds in your head and fire a second. Once the first missile hits slew the cursor to the seconds target. Since you fired the missiles with a 4 seconds interval (or, whatever interval you want), you know the second missile will impact in 4 seconds, therefore, you have 4 seconds to slew the cursor to the seconds target(it will hit as long has it is not too far away for the missile to maneuver).
Remember to turn the laser on. Good luck!