There's a lot of talk about Maverick's block system here so I figured I'd chime in.
The only time a rollback can happen in the tunnel is in the event of a power failure or ride/E-stop when the train is mid-launch. This is the ONLY time a rollback will occur. If the block ahead is not clear (called the "Pre-Waiting" block") the train will stop in the tunnel before the start of the launch. This is different than the lift, as the lift requires the train to pass that same "stop" point in the tunnel, and it is possible to send the train from the "Pre-Lift" block before then (possible as soon as the previous train crests the lift).
The train in the tunnel does not adjust where on the track it launches, it launches at the same spot (start of the magnetic launch motors). It cannot roll to the end of the tunnel slowly, it's actually a 20 foot incline from the start of the launch segment to the point the launch disengages, against very power magnets... not possible. The difference you might be feeling is from the kicker wheels before the launch. If the train is too slow or completely stopped heading into the launch, the kicker wheels will engage resulting in a boost of speed to hit the launch itself. During normal operation, the kicker wheels aren't felt.
(If anyone has a question about my source: I worked the ride in 2007, so I'm familiar with both the operations, the block system, the programming, etc. I know that ride like the back of my hand)