I am going to assume that Failover Mode 4 is being used which enabled ALUA based LUN access and an Active/Active configuration of the storage processors. I am also assuming 4 paths per LUN/datastore from each ESX host (2 HBAs, 4 array ports).
Let's assume I have VMs on a LUN that is owned by SPA. All paths will show as Active and the two going to SPA will show Optimized, while the other two going to SPB will be Non-Optimized. If I issue a trespass command to the LUN, then then the optimized paths will then be attached to the SPB paths and all IOs will process down these paths.
With MRU, a trespass of a LUN will change the Optmized/Non-Optimized setting per path and will cause IO to change to the first available Optmized path. This is very similar to RR, except in MRU there is not balancing between Optimized paths.
With FIXED, there is a preferred path that is set when the ESX host first talks to the LUN (or manually set), let's say to SPA (the currnet owner). If I trespass the LUN, the ESX host will continue to send IO to the Non-Optimized path and the storage processor will then send the IO to SPA through internal CMI bus. There is then an algorithm which will force the trespass of the LUN back to SPA if IOs continue to come in on the Non-Optimized path.