Replace integer arithmetic with more complex expressions, encoded with Mixed Boolean Expressions (MBA). For example, the following identities can be used to encode integer addition:
x + y = x  ¬ y  1
= (x ⊕ y) + 2·(x ∧ y)
= (x ∨ y) + (x ∧ y)
= 2·(x ∨ y)  (x ⊕ y)
For example, Tigress might replace
z = x + y + w
with
z = (((x ^ y) + ((x & y) << 1))  w) +
(((x ^ y) + ((x & y) << 1)) & w);
Here's a LigerLabs video that discusses this transformation:
Option  Arguments  Description 

Transform  EncodeArithmetic  Replace integer arithmetic with more complex expressions. 
EncodeArithmeticKinds  builtin, plugins  Specify the types to encode. Currently, only integer is available. From version 3.3.3. Default=builtin.

EncodeArithmeticMaxLevel  INTSPEC  How deep to recurse into expressions. Default=1. 
EncodeArithmeticMaxTransforms  INTSPEC  How many transformations to perform on each expression. Default=1. 
EncodeArithmeticRepeatTimes  INTSPEC  How many times to repeat the rewriting process. Equivalent to calling EncodeArithmetic multiple times. From Version 3.3 Default=1. 
EncodeArithmeticDumpFileName  string  Name of Json file onto which we dump transformed expression. The actual file will be functionname_number_fileName.json. From version 3.3.2. Default=100. 
There are two parts to attacking an MBA expression: first you have to find the expression, and then you have to decode it (i.e. turn it back into something resembling it's original form). You can split up expressions to make them more diffiult to locate in the code (both statically and dynamically).
Option  Arguments  Description 

EncodeArithmeticMaxSplit  INTSPEC  How many pieces in which to split the transformed expression. From Version 3.3 Default=1. 
EncodeArithmeticAddImplicitFlow  BOOLSPEC  Add implicit flow to the pieces split out from an encoded expression. You need to set EncodeArithmeticMaxSplit=value greater than zero for this to take effect. From Version 3.3 Default=false. 
EncodeArithmeticImplicitFlow  SExpression  The type of implicit flow to insert. See AntiTaintAnalysisImplicitFlow for a description. Default=none. 
EncodeArithmeticAddOpaques  BOOLSPEC  Add opaque predicates to the pieces split out from an encoded expression. You need to set EncodeArithmeticMaxSplit=value greater than zero for this to take effect. From Version 3.3 Default=false. 
For each operator, there are many possible encodings, and at transformation time, these are selected from randomly.
There have been many recent papers on attacking MBA expressions. To facilitate such attacks you can dump all the transformed expressions onto a Json file for further processing. Simply set EncodeArithmeticDumpFileName=filename.json.
Currently, the identities are taken from the book Hacker's Delight.