⺼ is the meat-flesh radical 肉 in left-side form (visually identical to 月 but distinct). It marks 胁 as a body-part character, joining 胸背肩腰 - all anatomical sites on the trunk.
办 here is a simplified graphic stand-in (the traditional 脅 used 劦 - three 力 stacked, depicting many forces pressing). The reform replaced 劦 with the simpler 办, preserving the visual silhouette while losing direct phonetic transparency.
