this couplet is quite straightforward. here zarb-e-yadullahi is used as a metaphor for holding fast unto the prophet's sunnah. sallallahu `alayhi wa sallam. this is derived from the verse: yadullaha fawqa aydihim / Allah aids those hands [which have sworn fealty upon the prophets] the hands here are of those companions who did bay'ah or swore fealty to RasulAllah. and a tangential reference that by doing so, they promised to fight - whether their own nafs or the enemy on the battlefield. and sajdah-e-shabbiri is a metaphor for sacrifice, martyrdom or intense worship. because sayyiduna Husayn raDiyallahu anhu, also known as shabbir was martyred when he bowed down to prostrate - and even in karbala, in such intense trial, he did not forsake prayer. so the couplet means: there are only two gems for a muslim that of following the prophet and worshipping Allah at all times or there are only two gems for a muslim the enthusiasm to strive and the spirit to sacrifice. fa'afham wAllahu ta'ala a'alam.