Impulse implemented double shoulder connections across its entire fleet of tools to limit tool failures. With ACME and API connections the nose doesn't shoulder up against anything. As you apply more torque, the connections want to keep going past the outer shoulder. When the threads climb past the shoulder the box splits. The double shoulder connection shoulders at the top and the bottom. As you apply over-torque, the connection has nowhere to go. It's housed between the two shoulders. See Photo 1.