This is a dependency node designed to take two transforms and keep a fixed distance between them. With one being the target, it will pull or push the constrained transform to maintain that distance. This node does not provide any orientation, only position constraining.
This node has several uses but in particular, it works well to get the effect of a car pulling a trailer or a wagon.
More information and a demo video can be seen here: http://www.scottenglert.com/projects/maya-plugins/sepushpullconstraint
This plugin includes a dependency graph node as well as a MEL command to setup the constraint.
About This VersionVersion 1.2.0, 5/25/2016
Added more improvements to the node.