Keep your Match Value a secret!
For common rules you can define a so called Match Value. For example:
This rule sais: include the pricefield only if 'abacadabra' is entered. The Match Value in this is 'abacadabra'.
2. Secret stuff
Sometimes this Match Value must stay completely hidden for your visitors. When the Match Value is not supposed to be exposed at all, enable the feature Protect this value. Then you are sure your Match Value will stay a secret. From now on, this rule is validated serverside.
The side effect is that the calculation of this rule will have a small delay, because it has to be validated serverside. Therefore this feature is not enabled by default.
See an example for how to use this feature to create a secret coupon code in your form: