Doubts about forming a crust on a souffle to make it rise further

I think rather than a crust what this process would ideally do is to seal the top a bit so it would trap more steam but still be flexible. The trick is to do it long enough to form that seal but not long enough that it forms an actual crust which would stick to the rim and inhibit rising. A seal forms on the top naturally in the first few minutes in the oven, I think the point of this technique would be to form that seal before baking to prevent steam from escaping before a crust would be formed, maximizing the rise. Whether it would work or not would I am not sure, I would suspect that the souffle ingredients, type of dishes used, and whether the rim is buttered or not would all be factors. The only way to know is to try it both ways and measure the results. If I had to put money on it I'd bet that this technique would get more rise, not by that much though.

If I was going to try this technique I'd generously butter the rim and side of the souffle dish and carefully monitor the top as it is in the broiler, pulling it out before it forms a real crust. I'd also have a knife or maybe a thin barbecue skewer ready to run around the edge to break off any that has stuck from the side of the rim.

Now if you want to be really slick, chefy, and slightly geeky you'll use a chef's torch instead to form the seal on the top, that way you can be sure it won't touch the sides and you can control the result completely.

If you do try this please post the result, I'd be very curious to know how it works for you.

I suspect the premise of your question is incorrect: broiling the souffle does not help it to rise in the slightest; instead, it gives it a brown, delicious, and visually attractive top:

(Image from The Kitchn)

Broiling (or grilling, in UK parlance works) is nearly 100% a radiant heat method: the heat is transferred to the food via infrared radiation, a slightly longer wavelength than visible light, but otherwise exactly the same. The radiant heat only directly heats the surface of the foods it shines on.

In the case of the souffle, which is in its souffle dish, this means only the top of the souffle can become browned. It gets a gorgeous color, and a brown, toasty delicious flavor without over baking the entire souffle.

Then, during the normal baking period, the souffle rises lifting the crust on top like a ship on a rising tide. The interior of the souffle which was not browned shows around the edges of the browned top, providing a delightful visual contrast.

The broiling must be done first, prior to the main baking, for a couple of reasons:

• If the souffle had risen, the contrast of the browned portion to the non-browned portion would be much less dramatic, since the radiant heat would reach some of the edges which are above the rim of the souffle dish
• Once a souffle rises to its maximum proper height, it should be served immediately. Otherwise, there is a risk of either collapse or overflow and rupture, neither of which are very productive

The crust is simply the outside of the food, which has been directly exposed to the heat sources. Because it is as the surface, it can dry out via evaporation of the surface water.

If the surface water is evaporated faster than it can be replenished from the interior of the food, the surface can reach temperatures above the boiling point of water. If it gets hot enough, caramelization and the Maillard reaction can proceed, giving the crust brown and toasty delicious flavors.

The crust has a different texture from the rest of the food because it is desiccated, and because different chemical reactions have occured due to the higher temperatures reached.

See also: Souffle article at The Kitchn describing the technique, based on a recipe from Paule Caillat

I know that if a crust forms on top of a souffle or cake, it will lose its ability to rise (not flexible anymore).

I think that you got this part wrong.

A cake or souffle indeed stops rising when the proteins in them set, so the batter is not flexible any more. But it happens when the batter is set in the area which is supposed to rise. Recipes and baking instructions for cakes are designed so that crust formation on top and the setting inside happen at the same time, because then you have a nice, even cake.

If you have ever baked a cake shaped like a small hill, you will know what happens when this fails. The upper crust of the cake is set and cannot expand. The sides of the cake are already inflexible or semi-flexible and do not rise (much). The center is still completely flexible and continues rising, creating an unsightly buckle and cracking the upper crust.

But a souffle rises differently from a cake. There, you want to have a vertical rise only. This is why you use a ramekin. A ramekin is straight-sided, and it has thick, insulating ceramic walls and a smaller diameter than a cake pan. This ensures that during a normal baking process, your souffle won't get a liquid center and set walls in a horizontal cross-section. The vertical walls covered with generous amounts of butter plus a non-pasting granulate (frequently grated parmesan for cheese souffles, nut flours for nut soufles, sometimes breadcrumbs) as opposed to flour for cakes, give you perfect vertical expansion while not permitting sideways expansion.

As SAJ14SAJ mentioned, a grill/broiler heats in a very specific way. It heats surface very quickly to very hot temperatures, but the heat does not really penetrate well under the crust. (It does so, but very slowly. This is why grilled chicken has to be rotated all the time, else its skin would burn hopelessly by the time the inside is cooked). By placing the souffle under the grill, you cook the crust only, to a very small depth. The crust itself cannot rise anymore, but this is not tragic, as the depth of the crust will be very small in comparison to the complete height of risable batter. The batter below it will not set. It will not even become semi-flexible. It will remain raw and perfectly risable. When you then put the souffle into a normal oven with bottom heat, the raw batter will be able to rise normally. It will just push the hard crust upwards.

So, the theory says that the trick can work well. I have not tried it myself and cannot say whether it has a noticeable effect. This will depend on how much steam is lost through the raw batter surface when the souffle is baked normally.

I hope that this clears up the question about why it can function for rising. The ideas provided in other answers, such as the tasty crust, are a further reason to try the technique.