Whilst the name may put some people off, the skull crush is the name given to the lying triceps extension which is commonly performed using an EZ bar (which is often more comfortable on the wrists and convenient than a barbell), but can also be performed with a set of dumbbells.
By isolating the extension of the elbow, the skull crusher stimulates the triceps – the three headed muscle located to the rear of the upper arm. Most people will find a shoulder width grip most comfortable (inner grips on the EZ bar), but it is worthwhile experimenting to see which feels most comfortable and effective.
Alternatively, you can perform the skull crusher using a cable pulley by placing the bench close to a cable station and attaching a suitable handle attachment. Not only will this ensure you receive maximum tension throughout the full range of motion (ROM), you can also experiment with many more grips by using different attachments, such as rope and V handles.