Either the spares have a higher chance of
being spared if they back-up and if they use their strikes for
spares on the that should be cause the pins left are on their left
side for right handers and right side for left handles.
Other than that pretty much all the spares done by pros i've
seen use normal plastic balls or high friction balls (which i'm not
sure if they are reactive a not)
So if i'm wrong or you think i'm a wannabe please just tell me
so I can learn new stuff :)