Cautious Speed is the mecha regular walking speed, the tactical movement speed in other words. It is the base speed of the mecha from which you can calculate the other speeds (air, water, running, etc).
A mecha in cthulhutech is an exoskeleton, your character uses his actions to pilot the mecha, and the mecha duplicates your actions accordingly, like in the mecha anime that inspired cthulhutech (ie: evangelion). This means that when your character takes his movement for his turn, so does his mecha, and when he uses his action, the mecha acts in return. All movement types and rules for characters on personal scale are also available to mecha scale.
About acceleration, the mecha chapter says:
This entry shows you the ways in which the mech can move, including its ground speed, any assistance systems like Jump Pods or Grapplers, any enhanced movement, and the like. Speed is expressed in miles per hour (mph) and in maximum/cautious yards per five second turn (ypt); jumping distances are shown in average long jump/high jump in yards. Acceleration is expressed by Code, and by turns necessary to accelerate to top speed and decelerate from top speed.
Which means that, if your mecha top speed is 100 and it has a Code 2/2, you must move for two turns before you can either go from 0 to top speed, and from top speed to immobile.
When you are running at top speed and make contact with another target (be it a vehicle, mecha or giant beast), you use a special mecha action called Charge Attack. On top of gaining a charging bonus on your attack, you also inflict damage on your mecha:
Barreling into something at top speed is both a good and a bad thing – it’s going to cause more damage to the thing you hit but it’s also going to cause damage to you. In order to make a charge attack of any significance you must be moving at more than 50 mph. If traveling at 51 to 100 mph, a successful charge attack adds two dice to the overall damage on top of success, weapons, and strength if applicable. Traveling at 101 to 200 mph increases the bonus to four dice. An unsuccessful charge attack simply means you fly right by your target.
On the other hand, you take half the total damage from the charge attack rounded down. Armor applies as normal.
Now, wether or not you make this attack against your target, you may decide to continue moving. This calls for the Break Away special action, which requires a d10 roll and a success on 6-10. If you fail, you cannot disengage your target that turn.