using macbook track pad box de select

ive just got a new macbook pro and am trying to figure out how to use the trackpad with 3 button mouse emulation. its coming along pretty easily but the one thing i cant figure out is how to box de select. with a 3 button mouse you would hit the b key and then hold down the third button to de select. i cant figure out how to do this with the trackpad…

hold option, duh