A squeeze catch can occur in a siteswap when a throw of value N is followed M beats later by a throw of value (N - M), with both throws directed to the same hand - for example, a 4 and a 4x thrown from different hands at the same time, or a 3 thrown right after a 4, or a 7 with a 5 two beats after it.
In vanilla siteswap, patterns with collisions like this are invalid, but it's possible to write valid patterns with squeezes using multiplex notation, by writing the next throw made by the hand that did the squeeze catch as a multiplex throw involving the same number of objects that were just caught by that hand. (This "multiplex throw" can include a 2, which would usually mean that one of the objects stays in that hand while others are thrown, so it may not be an actual multiplex throw.)
The time-reverse of 5 ball splits - ([44x],2)* - is the squeeze pattern (,4x)*, where the 4 from one hand and the 4x from the other hand land at the same time in the hand that threw the 4. Siteswaps like this can also be done without using actual squeeze catches, by throwing the objects to slightly different heights so that they don't land at the same time. Stacked patterns like , , and  are normally done with the stacked throws staggered so that the objects are caught one at a time.