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"Virtual Shielding" was developed as the demand for being able to carry out more ships functions without lowering defenses grew. It incorporates idle shield generators in a network routed through a central control system linked to the subspace sensors. As soon as sensors detect incoming weapons fire, the shield generator in the area which the attack is targeted activates, blocking the shot, and then deactivates again.
The primary advantage of virtual shielding is the ability to transport without lowering your defenses. It can also be useful because weapons fire which would miss the actual ship but is within normal shield range will inflict no damage, and also an observer cannot determine whether the shields are on or off, making raising your shields a precaution you can take without insulting another party. Shield energy is also conserved because it is not necessary to maintain full shields all the time.
Drawbacks exist, however, mainly in the form of reaction time. Torpedoes travel significantly slower than the speed of light, but phaser or disruptor blasts travel at light speed. Although the sensors used to detect these use signals which travel faster than light and can therefore still yield advanced warning, the reaction time to activate the shields is such that phaser or disruptor blasts which originate within 100 kilometers of the ship can get through the virtual shielding. Also, if a torpedo were to approach the ship at warp speed (relative to the ship), it would have to be detected at least 0.05 light-years away to get the shield up in time.
It is also impossible, using current technology, to overload virtual shielding with warp power because the generators designed to be overloaded with warp energy must constantly vent it to prevent an overload explosion. (see: Warp Shields)
~Jeremy Lennert, Simon Dark of the 7th Fleet