Headloaders are balls on the end of rods about 6" long and they fit where your blades go, so you can tune your engine without the heli lifting of the ground. The balls simulate the load that the blades would give. You can give full throttle and tweak your main engine needle.

Goto http://www.revolutionmodels.co.uk you can see what they look like under 'General accessories'

Very handy tool.