The pǎi reading is bound to one word, 迫击炮 (mortar). The pǎi here originally rendered the foreign gun name by sound. Taiwan keeps the regular pò reading for this word; mainland standard uses pǎi.
辶 is the bottom-left walking radical, the side form of 辵, drawn as a foot on a winding path. It is the indexing radical and supplies the meaning: 迫 is to press in on someone by coming up close. Same motion family: 逼 to force, 追 to pursue.
白 at the top-right supplies the sound, bái drifting to pò through the regular b/p alternation in this phonetic family. The white meaning does not carry over. The same phonetic gives 拍 to clap, 怕 to fear, 泊 to moor.
