勃 is rarely used alone. Common compounds: 蓬勃 (péngbó, vigorous/thriving), 勃然 (bórán, suddenly/agitatedly), 勃勃 (bóbó, exuberant, as in 生机勃勃 shēngjī bóbó, 'full of vitality').
Right strength radical, the indexing component (Kangxi 19). Pictures a flexed arm or plough. In 勃 it adds force to the 'sudden flourish' of 孛, giving 'vigorously arising, bursting forth.' Same radical anchors 力, 努 (effort), 助 (help), 动 (move).
Left 孛 supplies the sound: bèi shifts to bó with a tone change. Standalone 孛 means 'comet' or 'sudden flaring' — and that meaning fits perfectly with 勃 (suddenly arising, vigorous, abrupt). So 孛 functions as both phonetic and semantic. Same phonetic in 悖 (contradict), 脖 (neck).
