The response system is what makes the yo-yo return or 'wake up'.
How the response system works
When you throw the yo-yo, the grip of the response system on the string is what transmits the force from the string to the yo-yo body to start the yo-yo spinning as it falls. A yo-yo with little response may feel like it 'slips' as it falls due to not having enough grip on the string to make it start spinning sufficiently.
The response system also affects how the yo-yo returns. Some yo-yos return at the slightest tug on the string, some require a rather sharp tug, and some do not return at all unless you wrap the string around the axle (known as a Bind). Yo-yoers refer to yo-yos as having good, little or no response.
To understand why a response system is so important, let's first discuss what happens when the yo-yo returns. It's not so much the tug itself, but the moment that the string is slack for after the tug. During this moment, the string starts to wind around the axle two to three times until it is firmly wedged between the halves of the yo-yo. From that point on the momentum of the yo-yo is transformed to vertical movement up the string. This is why a yo-yo that is still spinning but only slowly will not return.
Types of Response Systems
During the first moment of slackness however there has to be some sort of friction, that causes the string to make those crucial first windings around the axle. The following section describes the different ways to achieve this:
In fixed axle yo-yos this friction usually comes directly from the axle. This is enough to tightly wind up the string. Plastic yo-yos with fixed axles often combine this with a starburst.
A starburst is a star or asterisk shaped figure around the axle, that slightly protrudes from the inner wall of the string gap. It is meant to provide a good grip, once the string touches it. The disadvantages of a starburst are, that they are sometimes noisy when the string touches them during play, and that they can wear out with time if the plastic is too soft. The Team Losi Cherry Bomb was notorious for this. Once the starburst is gone, there is no way of replacing it. Another problem is that it is very difficult to add these to aluminum yo-yos, because you can't create them on a lathe.
While a starburst protrudes into the yo-yo's gap, the reverse starburst, patented by Playmaxx, is an arrayed series of radial grooves or cuts that are recessed into the inner surfaces of the yoyo. This made for a much milder response system that eliminated many of the disadvantages of traditional starbursts. It was also a much easier modification for the average user to implement to his own yoyos as a sharp cutting tool was all one needed to add radially spaced cuts or grooves into the inner surfaces of both plastic and metal yo-yos. The original Henry's Viper was a common recipient of just such a modification.
Playmaxx refined the reverse starburst by adding rubber pads with arrayed "teeth" that encircled the bearing. This Brake pad technology (BPT) was also patented by Playmaxx and was one of the first modern systems that used replaceable pads/sticker. The rubber pads (resembling cork) with adhesive backing are set into the yo-yo halves, and provide an additional level of friction to catch the string thus providing responsiveness. The brake pads eventually lose their response over time and need replacement, and the rough surface of the pads prevents the yo-yo from being quite as smooth on the string as some other response systems.
Though patented in Great Britain in 1984, friction stickers did not gain real popularity until the late 1990s when the number of aluminum yo-yos being produced increased dramatically. Unlike reverse starbursts which are permanent and Brake Pads which require a yo-yo machined to accept them, these thin circular sheets of rubber or cloth can be applied to the inner surfaces of almost any yo-yo to increase responsiveness and then, if desired, removed. Major advantages of these is that they can be replaced when worn out, or exchanged for a different kind of material, if you do not like the feel of the particular kind your yo-yo came with. You can even mix different kinds on one yo-yo.
They can be either mounted on the inner wall or recessed into it.
O-rings are rubber rings that sit in a groove around the axle. Their function is basically the same as that of a friction sticker but last much longer.
Silicone has gained widespread popularity during the last years as a material for response systems. It is available as liquid silicone or as ready made friction stickers/o-rings. While the stickers or rings just replace their rubber counterparts, liquid silicone is usually injected in the recess and left to dry. Afterward any excess is shaved off using a sharp knife. Yo-yos with recessed silicone are generally considered to be for advanced players who enjoy very little response, and use a bind to return the yo-yo.
Hybrid Response Systems
The term hybrid is used for a response system that mixes any of these systems, most notably the mix of one O-ring and one starburst made popular by YoYoJam.
Methods of modifying response systems
One problem with most response systems is that they protrude from the inner wall (left image) of the yo-yo and thus sometimes rub against the string, slowing the yo-yo down. To get around this, some manufaturers have started to make little cavities into the yo-yo gap, so a response sticker can be mounted flush with the rest of the wall (right image).
For existing yo-yos it is also a very popular modification (or mod) to add this little recess using a lathe. There are even professional yo-yo modders that sell recessed versions, like Takeshi Kamisato's recessed Freehand Zero.
Homemade Response Stickers
Dropped in popularity since around 2005, people would often use brake pads and would cut them out, thus creating friction stickers. People have since stopped using them as silicone response has been more widely used.
This is a mod to achieve a recessed response system on an O-ringed yo-yo. It is done by simply shaving the protruding part of the rings off until they are flush (or nearly flush) with the inner walls of the yo-yo.
Add thick lubrication
Transaxle yo-yos often have very little response. One of the first approaches to solve this was to slow them down by applying a thick lubricant (thick lube) to the bearing. This (usually combined with another response system) makes for just enough resistance to make the string wind up until it grips and wedges itself into the yo-yo sides. The downside of this is that the yo-yo does not spin as long due a faster rate of deceleration of the yo-yo spin speed.
One very interesting yo-yo in this regard is the Mondial, which has a special hole in the axle through which you can pump thick lube using a syringe without having to unscrew the halves.