Carbenes add to double bonds to form cyclopropanes, and, in the presence of a copper catalyst, to alkynes to give cyclopropenes.  Addition reactions are commonly very fast and exothermic, and carbene generation limits reaction rate. 
In Simmons-Smith cyclopropanation, the iodomethylzinc iodide typically complexes to any allylic hydroxy groups such that addition is syn to the hydroxy group.