As the Demoman, the "Chargin' Targe" and the "Splendid Screen" have a charge attack which is activated with alt fire. While in charge attack, the Demoman normally can not turn left or right unless you have "Ali Baba's Wee Booties" or the "Bootlegger" equipped. Then you should be able to turn slowly.
Playing with the Hydra however, I am not able to turn in any direction but have to run a straight line.
The crosshair is fixed to the center of the screen during the whole charge attack and the game does neither react to analog stick input nor to aiming with the right controller.