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The NASA Spaceflight article China conducts first Sea Launch mission with Long March 11 launch of seven satellites links to the YouTube video China’s first sea launch: Long March-11 launches from a ship at sea.

The article says:

The Long March-11 (Chang Zheng-11) is a small solid-fueled quick-reaction launch vehicle developed by the China Academy of Launch Vehicle Technology (CALT) with the goal to provide an easy to operate quick-reaction launch vehicle, that can remain in storage for long period and to provide a reliable launch on short notice.

LM-11 is a four stage solid-fueled launch vehicle equipped with a reaction control system on the fourth stage. The vehicle has a length of 20.8 meters, 2.0 meters in diameter and a liftoff mass of 58,000 kg. At launch it develops 120.000 kg/f, launching a 350 kg cargo into a 700 km SSO. The CZ-11 can use two types of fairing with 1.6 meters or 2.0 meters.

Question: The first half-dozen seconds launch looks a little unusual to me. Once it clears the tower the rocket appears to hesitate slightly, then a sudden bright burst of exhaust happens and at the same time it looks like it starts accelerating much more quickly.

Am I imagining things, or does the Long March-11 increase its thrust after clearing the launch tower?

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Long March 11 is a cold-launched rocket, based on a concept popular with military rockets. A gas charge is generated by the launch platform (often using a chemical gas generator) to eject the rocket from the launch platform before engine ignition. This saves the launch platform from having to manage hot exhaust gases, and reduces wear and tear on the launch platform. This is popular in military platforms so they can use high thrust solid rocket motors to rapidly fling rockets into flight (and away from the ground where they might get struck by counteract) without overbuilding the launch structures to handle the thrust of those engines. LM11 likely uses military style hardware to A) Reduce launch vehicle design costs and B) demonstrate the proper function of military hardware in a non-aggressive context.

So, short answer - yes, the thrust increases from zero to 1,200 kN in the moments after launch.

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    $\begingroup$ Right, it's pretty evident that it's launched from a barrel-like tube. It also looks like something falls off the bottom of the rocket immediately before engine ignition. That could be a piston used for the initial ejection. $\endgroup$ – TooTea Jun 5 at 16:19
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    $\begingroup$ US sub launched ballistic missiles use a similar system to launch from just below the surface. $\endgroup$ – Leliel Jun 6 at 1:10
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    $\begingroup$ @TooTea exactly. This is nothing but a "cold launch" ICBM. $\endgroup$ – RonJohn Jun 6 at 1:16
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    $\begingroup$ @HarryJohnston There aren't many ways for a solid motor to fail to ignite. You can use as many redundant igniters as you wish and you only need one of them to work. There's no complicated startup sequence as in liquid engines; once the solid fuel lights, the rocket will go somewhere. And in case everything fails, a big heavy but most importantly cold chunk of stuff comes crashing down, which is likely preferable to a similar chunk of pure inferno. $\endgroup$ – TooTea Jun 6 at 11:15
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    $\begingroup$ @HarryJohnston, the design is for sub-launched or underground bunker launched missiles. Space is sometimes used as a proxy for nuclear war, to demonstrate to potential enemies just how accurate your nukes can be, reliable your launch system is, and how resistant to first-strikes they are so that they can take part in a second strike. All three preserve the MAD doctrine. Subs are particularly resistant to first strikes, so demonstrating that you have the ability to launch ICBMs from a sub is critical to any nation with nuclear weapons. $\endgroup$ – Ghedipunk Jun 6 at 17:58

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