No/confusing info available on use of "Event Wait" Flow node

Use of "Event Wait" (a node type exposed in the Eclipse IDE) is undocumented.
In particular, the content of "constraint" is confusing at best.
The IDE (in 5.0 - I haven't checked 5.1) has a Java/MVEL pulldown in the constraint editor, but the one example I can find, "ClinicalPathwayX.rf", has a constraint that reads
<constraint type="rule" dialect="mvel" >Diagnose( )</constraint>
which suggests that it's a rule pattern ("mvel" to the contrary). The compiler blows up with no clues if an expression like "foo==true" is put in the constraint.

If imports are entered in the constraint editor, they are discarded.
This means that use of the WorkflowProcessInstance, and hence use of process variables, is not accessible as doc'ed in the "Constraints" section of the docs.

Apologies in advance if any of this is just noobiness.

If anyone wants to post workarounds for all of this, great! If not, I'll cross-post on the user forum in a few days.