First, I never liked working with principal bundles; vector bundles seem easier and more natural to me. Second, I never like thinking about abstract principal G-bundles. I prefer fixing a representation of G and viewing the principal G bundle as a reduced frame bundle associated with a vector bundle.
So let E be a rank k vector bundle and F the bundle of arbitrary frames in E (this is a principal GL(k)-bundle). Then GL(k) acts on the right on F. Given a subgroup G in GL(k), let FG be a subbundle of F such that if finFG, then so is fcdotg for each ginG.
The primary example is E=T∗M and FG is the bundle of orthonormal bases of the tangent space with respect to a Riemannian metric.
What is the critical property we want a G-connection to satisfy? Well, any connection allows you to parallel translate an arbitrary frame finF along a curve. We'd like the G-connection to be such that if finFG, then the parallel translation remains in FG. This leads to the right definition of a G-connection.
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