When an unvowelled mim comes before ba (ب), it is read with ikhfa shafawi. Mim is not pronounced as an entirely separate, clear stop, nor completely merged into ba. Its nasal character is retained while moving toward the ba.
«يَعْتَصِمْ بِاللَّهِ»
Hold fast to Allah. The mim before ba undergoes labial concealment.
«تَرْمِيهِمْ بِحِجَارَةٍ»
Striking them with stones. The mim before ba follows the same rule.
Concealment does not mean deleting mim. It is an intermediate recitational treatment between full clarity and full assimilation. The correct lip position is best learned through listening and direct correction.