So, I have to warn you in advance that we don't actually know for sure. If you want, you can stop reading. Or you can keep reading and learn some cool things about sign language, preaching, and how to "give someone the finger"...in 1644. ;)

The Museo del Prado, which holds one of El Greco's works besides the one linked by OP (also painted by El Greco) is only willing to say:

the solemn and rhetorical gesture of the right hand, which is not common in secular works by the artist although fairly frequent in his religious compositions,

On its timeline of art history, even the Met is silent.

The central problem is that none of the painters who illustrate the gesture left us little decoder sheets themselves. It's not a longstanding medieval pose like the one for preaching.

But that's boring, so let's have a little fun.

The fifteenth and sixteenth centuries in Europe saw the "revival" of the classical art of rhetoric. In fact, medieval scholars loved rhetoric; they just adopted a text they thought Cicero wrote (he did not) and adapted it to the ars praedicandi--the art of preaching. But in the 15th and 16th centuries, it was Very Important that things be from antiquity.

So now--and this is where the "we don't know for sure" part comes in--we have to skip ahead to the mid-17th century. From this era, we suddenly learn all about the rhetoric of gesture.

One of the more important/famous authors in this is John Bulwer, a philosopher and medical doctor who was very, very interested in the idea of hands/gesture as communication. (He's sometimes considered one of the ancestors of British Sign Language, or rather, the father of BSL research--the whole idea of a formal language of the hands for deaf/Deaf people in the west). However, his several works on the matter were targeted at speakers, in particular, how to use gesture while speaking so as not to confuse one's audience.

The gesture in question is found in Chirologia, where on page 155 of the printed edition; 190 of the archive.org version; you'll find gesture "P" is called "conscientur affirmo." Check the key on the previous page and it sends you to #52. That takes you back to page 88/121-22, thence:

To lay the hand open to our heart, using a kind of bowing gesture, is a garb werein we affirm a thing, swear or call God to witnesse a truth, and so we seem as if we would openly exhibit unso sense, the testimony of our conscience, or take a tacite oath, putting in security, that no mentall reservation doth basely divorce our words and meaning, but that all is truth that we now protest unto.

Bulwer goes on to say that this gesture of deepest affirmation is found especially among ancient Greeks and contemporary Ottomans.

So I guess here is, technically, the conclusion:

• It's gotta mean something
• We know what it meant to some people in the 17th century

On the other hand, now you know that medieval people loved Cicero so much they claimed he wrote books he didn't; John Bulwer was one of the more distant ancestors of BSL; and if you flip around in Chirologia a little, you can pick up all sorts of GREAT gestures to use subtly.

Coming at this from the perspective of a modern artist I'm honestly fascinated - and surprised! - to find that this hand shape is considered to have some manner of secret meaning behind it.

While I don't at all doubt that much of what was placed in works of this time period was heavily steeped in symbolism and meaning, my own relationship with the W finger shape is derived entirely from two points which were hammered into my head over the course of 6 years of studio art classes: 1. the geometry of aesthetics, and 2. hands are an incredible pain in the ass to accurately draw - to the point where many artists will deliberately find ways to hide them rather than have to deal with the effort. It's also worth pointing out that you can see many examples of this hand shape in art all the way up to modern times, and it is by no means exclusive to the given time period.

In short: it's a visually elegant configuration, which makes it pleasant to look at, and it's easy to make a hand look recognizable and correct by rendering it in this pose.

The thumb, index and pinkie fingers are the primary framework for the hand, and as long as those look right our brains will recognize that yes, this is a hand. Since the placement of those three digits are significantly more important than the middle and ring fingers, the middle two tend to get visually lumped together as one shape. Here is a page of hand drawings showing several examples of this in action.

So while it's still possible that there is deliberate meaning behind the use of this pose, as one artist to another the far more likely explanation is that these artists made their living through commissions (as all artists did throughout most of history until fairly recently) and would frequently resort to shortcuts that made the work easier, such as drawing hands in simple poses that required minimal effort to get right.