Flying sideways

Am sure this is an easy one - but what settings does one set to make the copter fly in a left to right and then right to left direction with nose pointing north

not a perfect answer, but you could try using a roi point.

What @Michael_Oborne says could be an option but, what about using WP_YAW_BEHAVIOR set to Zero value ( Never change Yaw)? In this case, what orientation will the copter hold?: the one at power ON, the one when arming, the one at take Off, the one at engaging the navigation mode, the one the copter is so long the rudder remains untouched (and may be updating to new orientations with rudder action and holding it once the stick is released?)

Does anyone know if this way to block the orientation should be used the way I wrote? or may be this function is used for other purpouses?

Self answered, it works perfectly.