Regenerative shielding is one of the newer technological developments in Starfleet, and was introduced on the Defiant-class in the mid 2370s. This was the Class-IB model, with the Class-I being the prototype aboard USS Limitless, NX-70135 (Nova-class). The principle of regenerative shielding is as follows:
A typical Federation shield provides defense against energy and kinetic projectile attacks. It is essentially an oval divided into six sections (fore, aft, port, starboard, dorsal, ventral), a series of non-intersecting arcs of protection. When a shield takes enough punishment, it is knocked offline and the generator overloaded. The shield generator must cool down and regain control of its projector matrix before being reinitialized.
Regenerative shielding uses intersecting arcs of protection. When an arc is damaged, one of two things happens:
A) Extra power, drawn from the warp core or other systems, is sent to the generator, in an effort to boost the shield's durability. If the shield is heavily damaged, the generator may be bordering on overload.
B) Extra power, drawn from the warp core or other systems, is sent to a different shield generator. Its arc is expanded, thus, the deterioration of one shield is compensated by the expansion of another.
Regenerative shielding also employs a limited feedback loop to ensure that event (A) happens at a steady pace, meaning as the shield take hits, it is continually reinforced by its parent generator.
This technique has been employed to great effect on the Sovereign-class starships, and many existing designs are being refitted to employ similar technology.