Sounds like TCS, (Touch Control Steering), that is in the DHC8. When the TCS button is depressed on the control wheel it uncouples the autopilot servos from the aircraft without disengaging the autopilot. When you let go it holds the current pitch attitude. Depending what vertical mode is engaged it is referenced to that, so it can be basic pitch, but more likely IAS, VS,or ALT. So if you want to change something without reaching for the AFCS controller you can TCS it. It also works if flying manually with flight director modes engaged. Instead of asking the non-flying pilot to increase rate of descent or change climb speed, you can hold the TCS button, change the attitude, let the button go and the FD now has new attitude. Another example of when you might use it is if an MDA is a 50'altitude, eg 850'. You would normally have the altitude preselect at 900', and once the altitude captures at 900', you then press the TCS, descend the aircraft to 850', let go and now 850' is maintained.
Many basic AFCS systems in light aircraft, like the King 150 (I think) also have a similar thing called "pitch sync" on the control wheel.