دَخَلَ الْعَامِلُ يَفْحَصُ الْمِضَخَّةَ.
The worker entered while inspecting the pump.
The affirmative imperfect yafḥaṣu can function as a circumstantial verbal clause describing the worker's action upon entry. A positive imperfect used in this way normally appears without an independent circumstantial wāw, and its concealed subject links the clause to the worker.
Adding wāw risks suggesting a new coordinate action or another construction. For this intended circumstantial reading, omission is the clearer conventional form; the syntactic relation matters alongside the verb's form.