In music, a jam session is a collaboration that arises from familiarity with a particular genre while experimenting with style and details.  It is a finely honed balance between order and chaos.  For example, while the notes chosen (and their sequence and timing) will be free-form, if someone hits a note that is outside the key currently being played, they will typically soon resolve back into that key, so as to maintain a unifying theme.  The character of any particular jam is influenced by genre, instruments, relationships among the players, and many other things.

In group work, we are using the term jam session to refer to a conversation that is less linear, less careful, more simultaneous than a typical meeting.  The purpose is to invite creative inspiration.  When the group feels stuck and needs new ideas, business as usual is not working, or the energy has become dull, those are occasions to consider invoking this pattern. 

With a driving principle of mutual inspiration, the dynamics involved are extremely rich in content and context.  Cues such as body language and eye-contact become even more influential than in a regular session.  Some would say that "jamming" creates a pathway for spiritual expression.  The chemistry among participants can strongly affect how much synergy emerges.

Variations

The technique most associated with this is brainstorming. There are other, less well-known methods of brainstorming such as the "card-storming" or "snow card" approach developed by the Institute for Cultural Affairs, where the ideas are written on separate pieces of paper instead of spoken aloud, then clustered by topic and organized.  The facilitation method most akin to a jam session is Dynamic Facilitation (and the most opposite is probably Roberts Rules of parliamentary procedure).  However, you can invoke this pattern without doing any of these methods, simply by inviting people to share openly, jump into the conversation without waiting for a turn, and not worry about keeping on topic.  Paradoxically, taking the pressure off to arrive at a useful product often increases the chances of actually getting one.

Cautions & Caveats

One thing to watch out for when doing this is that less assertive members may have trouble getting a word in.  Thus this approach may need to be balanced with calmer, more ordered approaches in order to garner the wisdom of the full group.

Another thing to keep in mind when guiding this process is that like a tender seedling trying to sprout, creative new ideas often need protection in order to grow.  Thus the guideline of "no evaluations" during brainstorming.  Sometimes even a question can feel too judgmental and throw someone off.  So consider carefully what ground rules or other techniques will create this container in your particular circumstance.