when in low (red lever back )its in 4wd at all times ,when in high (red lever forward )its in rear wheel drive only ,if you wish to select 4wd in high you just push yellow knob down this can be done when moving or stood still,to put it back to 2wd pull red lever back into low then forward into high yellow knob will pop back up then ,only use 4wd on surfaces were wheels can slip ,not on tarmac as in 4wd both ales are driven at the same speed and makes no allowance for cornering so transmission wind up will occur