It proposes that certain limitations on scrambling (reordering of clausal constituents) in German should not be treated as performance limits (in the way the limits on center-embedding in English usually are) because there is a way to make them follow from a grammar formalism (namely, TAGs), and this provides a better explanation than positing a performance restriction stemming from some unknown psychological or neurological basis.
the paper advocates letting the syntactic theory decide: whether a theory is available that will draw a certain syntactic distinction should be a relevant factor in deciding when and whether performance limitations are to be invoked.