It's disturbingly easy to use position ranked to climb to ranks you don't deserve.
Step 1: Find a duo partner with a different main position than you.
Step 2: Do placements in your Duo's main position, placing as low as you can. Have them do the same.
Step 3: Queue up with your duo. Set your preferred position to your Duo's position. Have them do the same.
Step 4: Get into a lobby. Swap positions with your duo.
Say you're a gold 4 Mid/Jg duo queue, each of which has Bronze 1 in their duo's position. If you queue as your Bronze position, you'll get matched with other Bronze players, but you're a Gold level player, so you crush them. Thing is, the game detects that you swapped positions, so you'll gain the +20 LP in your main role.
If you do this, you can gain LP in a higher ranked role from playing in low ranked games.
This is worse than Dynamic Queue.