DJL (Dave J Looker) is mainly known as the co-author of the Speedlock protection system, a fastloader used on many cassette games released in the UK, but he was also the author of several Amstrad games including 3d Stunt Rider and Nigel Mansell's Grand Prix.
Speedlock was a modified version of the Spectrum's cassette routines with protection added.
As the protection system continued to evolve, programs such as Soft-Lok and the Bonzo suite were upgraded to break the protection.
A separate disc protection system, 8k Speedlock, relied on the fact that the CPC's hardware could reliably read but not write 8k sectors. To get around this, the Executioner's utility Xexor included a 'brain file' solution to crack individual games before copying them.