Primary control is the hydrostatic pressure of the mud column, kept above pore pressure and below the fracture pressure at the shoe. Secondary control is closing the blowout preventer and circulating the kick out through a choke, holding a calculated drill-pipe pressure so the bottom of the hole stays just above pore pressure without breaking the shoe.
A kick is the unwanted influx. Well control is what is done about it. Circulating out at too high a pressure loses the well at the shoe. Circulating out at too low a pressure lets more formation fluid in. The procedure only works if the influx was noticed and the well was shut in while the mud column was still mostly in the hole.