Bring elbow of blocking arm to center line. Other fist goes palm down under it across center line. Lower arm pulls back to chamber. Blocking arm moves in an arc downward to side.
In kata often the arms spread wide before starting the block.
Fist ends one fist distance above hip. Practice also out of shiko dachi, where fist ends fist distance over leading knee.
This block is often used to redirect kicks, usually with some sort of side-stepping to get the body out of the line of attack.
Retracting (non-blocking) hand can also be used as a block of a strike to the ribs, after which the other arm sweeps it to the side.