I’m late to the party but I think I can address what the OP is asking about.
The short answer is: climb (or quiet) power.
The long answer:
When a jet (for our purposes let’s just say a typical airliner like a 737) prepares to take off from a runway there are multiple calculations done. The length of the runway, the pressure altitude of the airport, the temperature, the weight of the airplane and the wind are all considerations. Any obstacles on departure (radio towers or hills/mountains) also come into play.
In the old days a flight engineer with a bunch of spaghetti charts would figure out the thrust setting, takeoff roll distance, climb rate and decision speeds. Now this is all done by computers - either on the aircraft or in operations and then the numbers are sent to the aircraft. What you come up with are the thrust settings and speeds required to take off from that runway. This includes a Takeoff Power setting.
Jet engines are like any other mechanical things - the more you run them at their limits, the more likely they are to fail. They are also VERY expensive. So operators want to minimize the time the engines run at very high power settings.
For takeoff you will have a power setting that you must use to get you airborne and satisfy the initial climb requirements. Once airborne and climbing away you can safely reduce power to what is normally called “climb power”. This is a power setting that allows you to keep climbing but significantly reduces the stress on the engines.
The typical climb profile has climb power being set at 1000 feet above the airport you just took off from.
So when you are barreling down the runway you are feeling the acceleration under Takeoff Power. After safely airborne with the landing gear up the power will be reduced to Climb Power. The reduction in acceleration is what you feel as “slowing down”. You are not actually slowing down, but your rate of acceleration is.
The reason it is so pronounced is because the reduction from Takeoff Power to Climb Power is substantial and the airplane is still climbing - all of the lost “oomph” comes from the forward vector. When the airplane reaches climb speed (250 knots below 10,000 feet) Climb Power will still be set and the rate of climb will increase, resulting in almost no feeling in the cabin.
For those of you who want to see this in action - make sure to be listening to the engines right after takeoff. On widebody jets like the 767 or 777 you will be able to hear the power reduction better if you are sitting in front of the engines (ie in front of the wing). On smaller jets with smaller engines (737, MD-80, A320) your best bet is to be in front of and close to the engines.
Also, Merkwurdigliebe was pretty much spot on!
