The "mosaic trick" is a way to perform horizontal pixel doubling in hardware rather than software. And to do this trick, you turn on the SNES's Mosaic feature, scroll 1 pixel to the left every other scanline, and scroll upward one pixel after each two scanlines have been drawn.
Normally the SNES mosaic feature just the top-left pixel of a 2x2 square into that entire square. But the trick makes a different set of pixels get doubled horizontally on the next scanline.
It requires a different arrangement of pixels than the normal way of drawing tiles. A tile containing these pixels:
01234567
becomes this when viewed on two scanlines:
00224466
11335577
Actually performing these scroll writes does not require any CPU intervention because you use the SNES's HDMA feature to do those scroll writes.
Normally the SNES mosaic feature just the top-left pixel of a 2x2 square into that entire square. But the trick makes a different set of pixels get doubled horizontally on the next scanline.
It requires a different arrangement of pixels than the normal way of drawing tiles. A tile containing these pixels:
01234567
becomes this when viewed on two scanlines:
00224466
11335577
Actually performing these scroll writes does not require any CPU intervention because you use the SNES's HDMA feature to do those scroll writes.
User "93143" on Nesdev describes the Mosaic trick in this post: https://forums.nesdev.org/viewtopic.php?p=205633#p205633, other discussion here: https://forums.nesdev.org/viewtopic.php?t=20393&start=135
---
So now that you've done this, you need half as much video memory as before, which effectively doubles your bandwidth for rendering and transfers.