as fasteners or a weld referring now to fig4 and 7 the collar 342 is a dowel like structure having an arcuate portion 345 a shoulder portion 346 and a passage 347 when the button member 340 is in the first position the arcuate portion 345 protrudes out of the transverse bore 334 and the shoulder portion 346 partially blocks the hammer axial bore 332 as shown in fig7 the radial offset between collar passage 347 and the hammer axial bore 332 provides a mechanical interference that prevents the anvil 350 from entering the axial bore 332 of the hammer 330 referring now to fig8 when the button member 340 is in the second position the collar passage 347 radially aligns with the axial bore 332 thereby allowing the anvil 350 to enter the axial bore 332 via the collar passage 347 referring briefly to fig7 the trigger ledge 229 is positioned to impinge the arcuate portion 345 of the collar 342 when the trigger housing 228 moves in the u201c u u201d direction relative to the hammer 330 as can be appreciated the arcuate portion 345 acts as a ramp that facilitates smooth contact between the trigger ledge 229 and the collar 342 the interfering contact between the ledge 229 and the collar 342 forces the collar 342 to be depressed into the transverse bore 334 thereby moving the collar 342 from the first position to the second position referring now to fig3 and 4 the anvil 350 transfers the jarring force delivered by the hammer 330 to the tool string lower section 24 via the mandrel assembly 400 the anvil 350 includes a base 352 a reduced diameter sleeve portion 354 and a passage 356 the base 352 is preferably configured to.