An employee was standing on a ladder in a well cellar installing a tubing head with the assistance of a forklift operator. The tubing head was moved into place by running a 2-inch lifting strap through swivel hoist rings on opposite sides of the tubing head and around the forks of the forklift. The employee was removing the swivel hoist rings from the tubing head when the forklift operator started to boom in the forks of the forklift. This caused the strap, which was still being attached to the forks and the swivel hoist ring, to tighten and lift the swivel hoist ring housing. The employee's left thumb was caught and severely lacerated between the hoist ring housing and hoist ring cap screw.