Kadee 321
Code 100 Delayed 'between-the-rails' Uncoupler 321

Place this handy "delayed-action" Uncoupler anywhere on a straight track segment between-the-rails.
For Code 100 (2.5 mm) rails.
One pair with instructions. 

Delayed Magnetic Uncoupling®
1. Stop with the couplers over an uncoupler and back up slightly with the couplers still over the uncoupler, allowing slack to occur between couplers.

2. Pull forward bringing coupler off the uncoupler. Couplers will snap to the delayed position.

3. Back up, pushing the car(s) to the desired location. Do not permit slack to develop between couplers.

4. Pull forward, leaving the car(s) where desired. Couplers will automatically return to normal coupling position.

https://www.youtube.com/watch?v=A51sBEm6VFA