It's [http://source.contextgarden.net/tex/context/base/cont-new.tex?search=hangaround in the source].
<texcode>\starthangaround{\externalfigure[cow]}\input tufte\stophangaround</texcode><context source>\setupexternalfigures[location="yes">{local,default}]
\starthangaround{\externalfigure[cow]}
\input tufte