If you agree to the test drive what is offered, you will proceed to your chosen battle with the test drive item equipped, and you can to test it for as long as you want, until you decide to change your equipment or go to some other section of the game.
Once have finished your test drive, Tanki will offer you a discount on the item you just tested for a limited-time. The duration of the discount is limited to several hours and the size of the discount will be random. You might get a real bargain or perhaps only a small percentage off.
Personal discounts can be given even without a test drive too, and players will be offered such discounts randomly upon entering the Tanki Online game.
More importantly perhaps, personal discounts will combine with garage promotions and product kits plus more discounts given for fully micro-upgrading parameters of turrets and hulls.
The system for test drives and personal discounts is another way of getting equipment at favorable prices, providing you with a chance to test out a turret or a hull that you have not yet purchased to date.
It would seem that Tanki has looked at this development from the angle of getting players to increase their garage inventory, and that is something I can understand as the more equipment you have, the more upgrades you need to perform, and increase the probability that you will purchase crystals to do so.
I always try to look at things from a slightly different angle, and I see something that leaves further room for improvement and one that I hope Tanki will address. The flaw in the idea, as I see it, is as follows. You enter a battle with a turret and hull that the player has deemed suitable for that specific battle mode, and this can vary from map to map. For example, if you want to play Duel, you would use equipment that is suitable for close quarter battles and have a fairly quick hull to enable rapid movement to try an dodge enemy fire. The Tanki system then randomly offers you the option to use a Shaft turret, perhaps the most unrealistic turret to use in such circumstances. Another example would be a Twins turret on Monte Carlo, perhaps the least suitable for that battlefield. If you test drive Shaft or Twins under such conditions it is unlikely that you will buy it which defeats the whole idea of the offer. Similarly you may be offered a Mammoth hull when entering a battlefield where mobility is essential.
Another potential downside, and one that is not clear yet is....If you are offered an M1 turret or hull for example, it may not have any micro upgrades, which could leave you with a huge disadvantage in battle and again possibly leaving you with a less than positive test drive experience.
I do praise the Tanki developers for creating this facility as I do see some merit but as I suggested above, there is room for further and in my opinion, better development at a later date.