You are creating a slit in the fabric for the armholes. See the diagram on pattern. Beg with a WS row,
Start with the Purl side facing you work 5 rows in St st, p, k, p, k, p,
then cast on 30 (32, 34) sts above bound-off armhole edge
Knit K 52 (54, 56), cast on 30 (32, 34) sts, to fill the gap in the row where you cast off before. (Use Cable Cast On, or Single Cast on) and knit the 30 sts from previous section, to rejoin the other part of the row.
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