A grapple is a hook or claw used to catch or hold something.

A grapple can be mounted to a truck, tractor, or excavator with a movable arm that may lift, extend/retract, and move side-to-side (pivot or rotate). Some machines also have a separate control for rotating the grapple, also known a tiltrotator.

Simpler grapple machines consist of a hydraulically lift-able fork, rake grapple rake, or bucket and a movable, opposing thumb (one or more hooks or levers) that enclose and grip materials for lifting or dragging. 

A demolition bucket or multi-purpose bucket on a loader may also operate as a grapple in this application the bottom and rear side of the bucket are hinged and can be forced apart or together with hydraulic cylinders.

Steering clutch

Most older bull Dozers use a clutch and brake system for each track. The clutch disengages power to the track for gradual turns, applying the brake causes a sharper turn. This system works because there is no differential between the tracks. Some newer ones use one lever or pedal for both functions. 

The first part of lever or pedal travel releases the clutch, moving it further applies the brake. Another system is the controlled differential, it uses a brake on each side to cause power transfer to the opposite track thereby slowing one track down and speeding up the other. This system is widely used on military tracked vehicles, and some dozers. Oliver (Cletrac) used a controlled differential on the HG/OC-3 and maybe others. Caterpillar has also offered differential steering.  Most new machines have gone to hydrostatic drive, where a variable speed hydraulic motor is used to drive each track.