The synapsis is the pairing of a chromosome (two homologous chromosomes) during meiosis. Synapsis can increase the variability because synapsis and crossing-over (between two different chromatids) occur during meiosis phase 1. Crossing-over only occur during meiosis between two non sister chromatids. 
The two homologous chromosomes binds together during the Zygotene stage in the Meiosis prophase 1. The axial core on one homolog from each pair are linked together as transverse filaments forms between them- this is the Synaptonemal Complex .
Synapsis is completed in the Pachytene stage. The two chromosome pairs are help tightly together and thus become and behave as a single unit called a Tetrad . If this does not occur properly, it affects the rest of meiosis as there is no crossing over so genetic recombination cannot occur.