Let us understand that a skyhook is a "mind-first" force or power or process, an exception to the principle that all design, and apparent design, is ultimately the result of mindless, motiveless mechanicity. A crane, in contrast, is a subprocess or special feature of a design process that can be demonstrated to permit the local speeding up of the basic, slow process of natural selection, and that can be demonstrated to be itself the predictable (or retrospectively explicable) product of the basic process.
From Darwin's Dangerous Idea by Daniel C Dennett, 1995, The full argument can be read on pages 73-84.
Dennett's Skyhook argument/ joke is famous and popular among 'Skeptic'/ atheists - the sort of person who finds it endlessly amusing to refer to Christianity as a cult of the Flying Spaghetti Monster - as a put-down of religion.
However, if you review the argument and reflect on it, what it amounts to is a denial that metaphysical assumptions are necessary. That metaphysical assumptions are skyhooks, hence nonsense.
By calling them skyhooks, metaphysical assumptions of any and all kinds are being mocked as imaginary, arbitrary, impossible. incoherent, ridiculous.
By contrast, natural selection is put forward as a theory without metaphysical assumptions - here terned a 'crane': that is a theory that builds entirely from the evidentially-known ie. from science. A crane is therefore, is asserted not to be based on any metaphysical assumptions at all.
'Cranes' are an example of metaphysical denialism.
The assertion is made that there exists a 'crane' mechanism for progressive change that does not require any metaphysical assumptions; and - unlike a 'skyhook' a crane is real and actually works...
Whether Dennett truly believes that natural selection in particular, and science in general, are (somehow?) not built-upon metaphysical assumptions is unclear to me.
But I don't think Dennett really cares whether his argument is true; because his motivations are quite obviously, and gleefully, destructive of Christianity in particular and religion in general. To club them to death, any false argument is welcome.
In reality - Dennet must be ignorant, dishonest or evil - or some combination thereof. And Dennett's self-styled skeptik/ atheist fanboys likewise.
If we want to name-call metaphysical assumptions 'skyhooks', then everybody and all theories and all ideologies are necessarily hanging-from skyhooks all of the time - the difference is that some religious people recognise and acknowledge their assumptions, while atheists Never Do.