The "chuck mod" in a nutshell:  Does a 16-bit read instead of two 8-bit reads.  However, doesn't work on AT&T 6300/M24 because they get that wrong.  More from Chuck:

I'll add a bit here. The "Chuck mod" generally works when the host CPU is an 8088 or V20 (i.e. a CPU with an 8-bit bus interface).
Its operation depends upon the functioning of the 8088/V20 BIU. If you've got an 8086/V30/80286 CPU, then it's a roll of the dice--it all depends on how carefully the designers of your board decided to emulate the 8088 BIU. 

Most have it wrong--the Olivetti M24/ATT 6300, for example, does split a 16-bit read/write into two 8 bit ones, but gets the order wrong. I don't know about the Tandy 1000 286 systems, but I suspect they've also got it wrong.