It is a "Traverse Lock" when you grasp the handle the lever is pulled in, which disengages a small pin in traverse gears, a bit like in your car when you use "Park" with an auto gearbox. The reason for this is when traveling in a tank the turret will do its own thing and needs to be secured, also if the tank is parked on the side of a hill, gravity will pull the gun around until it points straight down the hill, not good if you enemy is at 12oclock and your gun is at say 9oclock if you are on the left side of a major feature[i.e.the tank is on a slope]. The early tanks, they didn't have a real problem with this as a man had the strength to traverse the gun uphill. Power traverse was added to the Tiger, not as a luxury but as an essential to aid the gunner for the above reasons. As Tank warfare escalated power traverse has taken over, merely for speed of acquiring the target and has left mans strength way behind.
We still have a traverse lock, now called a "Travel Lock" mostly used for transport purposes. In my early year as a tank crewman it was not uncomen to get into a veh. which had this pin broken, in these cases we would use a piece of wood shoved in between the traverse gears to stop the turret moving, crude but effective.
I hope this answers your question
Darren
KnightsCross