Speedlock is a tape and disc copy protection solution.
'8K Speedlock' refers to a specific version of the Speedlock where most of the data was stored on specially formatted tracks. Each track was setup with µPD765 sector size 6, which indicated a sector with 8,192 (&2000) bytes of data. However in reality, only 6,146 (&1800) bytes were useable. Recreating this format on the Amstrad was difficult because the µPD765 would try to write 8,192 bytes of data, and in doing so it would wrap around on itself and corrupt the sector data header thereby making it impossible to read.
Internally the format was known as "Indy", a reference to the fact that it was necessary to synchronize data writing with the index hole and use special hardware to avoid the wrap around. This synchronization was deliberately not maintained during duplication, thereby enabling data to load more quickly.