What will the soft reset do?

This is more or less correct. However, soft reset does actually effect your MMR as well as your rank. The 10 placement games then add or subtract a small amount from your MMR and placement. This is more or less a guideline:

Low bronze - Low bronze/Mid bronze High bronze - Low bronze/Mid bronze Low silver - Mid bronze/High bronze High silver - High bronze/Low silver Low gold - Low silver/Mid silver High gold - Mid silver/High silver Low plat - High silver/Low gold High plat - Low gold/High gold Low diamond - High gold/Low plat Mid diamond - Low plat/High plat High diamond - Plat 1 Master/Challenger - Plat 1

New accounts will place somewhere between Bronze 1 and Silver 3.

Given that this is my 4th year doing placements (although the system has changed a lot), you can generally expect to drop about a full tier. For example, my account is currently in Plat 4 and I would expect my placement to be around Silver 1 or Gold 5.

Lastly, I think the system places you slightly under the rank for your reset MMR. So after you finish your placements, you will be earning anywhere from 25-30 LP per game and losing around 15-20 LP. The reset is not meant to erase everything you have worked for, but to give you something new to work towards.

Also note that duoing can have a huge impact and massively effect your placement positively or negatively.