Yes the down stroke and fuel transfer is being done in this proces, but also, high speed fuel transfer is accuring as the case is being charged before the up stroke to TDC, at BDC the port is always open, as the up stroke starts, fuel that is traped under the piston is then releived through the port and charged into the cylender before the transfers and boost close, as the stroke down starts, the port reopens through the transfers and then the releife port.= a charge system thats efficiant and allows the largest boost of fuel and air possible.