When fucking a chick from behind, either through sexual curiosity, anger or just boredom you stick a finger into her anus. The Smithy is not limited to a single digit over time the goal is to work up to a Full Smithy (whole fist!).
The effects of The Smithy vary so be careful when using it.
Dude that bitch was so boring I did The Smithy to liven up the sex.
Mate smell my finger......... I totally Smithy'd your sister last night.