Lagann is the Breaking Drill Hex, as used several times during the Hogwarts army battles and during the fights between bullies and S.P.H.E.W. When cast, a spiraling blast of green light exits the wand. This blast will smash into shield spells, being specifically designed to weaken or destroy shields. Considering that first-years are capable of using it, it probably is not the most advanced shield-breaking spell, and when used by the first-years, it was not always capable of taking down more powerful shields in one shot. When used by more than one caster on the same target, the power is amplified and more effectively destroys defenses.
The incantation is a reference to Tengen Toppa Gurren Lagann, an animated Japanese TV show in which powerful drills play a large role.