A mechanical model is presented of precursory source processes for some large earthquakes along plate boundaries. It is assumed that the pre-seismic period consists of the upward progression of a zone of slip from lower portions of the lithosphere towards the Earth's surface. The slip front is blocked by local asperities of different size and strength; these asperities may be zones of real alteration of inherent strength, or instead may be zones which are currently stronger due to a local slowdown of a basically rate-dependent frictional response. Such blocking by a single, large asperity, or array of asperities, produces quiescence over a segment of plate boundary, until gradual increase of the stress concentration forces the slip zone through the blocked region at one end of the gap, thus nucleating a seismic rupture that propogates upwards and towards the other end. This model is proposed to explain certain distinctive seismicity patterns that have been observed before large earthquakes, notably quiescence over the gap zone followed by clustering at its end prior to the main event. A discussion of mechanical factors influencing the process is presented and some introductory modelling, performed with the use of a generalized Elsasser model for lithospheric plates and the ''line spring'' model for part-through flaws (slip zones) at plate boundaries, is outlined briefly.