How does the Pre-charge and Equalizing circuit shown in the below figure work?
Before every read/write operation, the Bit Lines must be pre-charged
  and equalized. This is done to assure that small voltages can be
  easily detected by the Sense Amplifier. The transistors Q8 and Q9 are
  responsible for charging the Bit Lines with VDD/2 while Q7 equalizes
  the voltage between them. The Pre-charge circuit is activated through
  the precharge input (φP).


Q7/8/9 act like voltage controlled switches (with resistance). When φP is active the FETs are turned on. Q8 connects ~B to Vdd/2, Q9 connects B to Vdd/2, and Q7 connects ~B to B. So effectively you have this:-







simulate this circuit &ndash; Schematic created using CircuitLab

The bus capacitances charge up (or down) to Vdd/2 through the Drain-Source resistance of Q8 and Q9, while current can flow in either direction through Q7 to equalize the voltages.