powerballs

ungreased gyroscope
is a great way to pump up spirals projections and wei chi work
working the arms without straining the heart

force control at very small rotation speed
can be basis for longer practise
whole arm/body work

but on other side
burns joints and tendons if forcing, misallingment
or going for high speed

most are good
e.g. gyrotwist trembles less then ordinary nsd powerball
and metal nsd powerball the least

effect is the same as rotating 10kg dumbell around its center that is fixed in a gyro frame