Boot9strap (B9S) is a modified firmware that makes use of sighax and a vulnerability in the console's NDMA engine to gain code execution on the ARM9 BootROM (Boot9) and load a payload (boot.firm) from the SD card. It uses a modified version the ARM9 payload loader of Luma3DS. The software can also dump the console-specific OTP and BootROMs.

B9S is a bootrom exploit that runs a lot earlier in the boot process in comparison to Armloader9hax (A9LH), allowing access to new system files that A9LH didn't, as well as adding better brick protection, and update protection, because it's unpatchable without a hardware revision (it is also worth noting that B9S uses .firm files instead of .bin files for payloads). B9S has effectively replaced AL9H, and if you use Luma3DS and would like to still be supported with updates, you have to switch to B9S.

For more technical details, refer to the presentation here.