Hi Paul, what aircraft are you flying? (useful to say upfront as all are not equal). Attitude (ATTI) mode can be activated manually (LHS switch on Phantom Remote for example) or if the drone looses sight of GPS satellites (say under a bridge) it may ‘drop into’ ATTI mode on its own. The built in barometer (measures air pressure) will maintain the aircraft height and the aircraft will fly just fine other that it no longer has gps info to maintain position, meaning the slightest breeze or motion will cause drone to drift just like a balloon in the wind. With practice, it’s not difficult to control this, and considered good pilot training to learn how to do this manually, so if it occurs unexpectedly, one can control and land the drone safely.
Edit. So yes, one can switch between ATTI mode and normal mode on most drones, again how depends on your drone & RC. (User manual would tell you).
Trust this assists you.