When you achieve an elite level (i.e. Silver/Gold/Plat) you will have the elite status for the rest of the year you achieved it and all the following year.

If you do not have enough nights to keep that elite level, then after your year + of elite status, Marriott will only drop you one level. You will then be that one lower level elite status for all the next year.

So for example, you had 75 nights in 2011. You would have achieved Plat for the rest of 2011 and all of 2012. If during 2012 you only have 4 nights, you won't have enough nights to keep Plat. You will only be dropped to Gold for 2013 even though you didn't have the 50 nights normally needed. This is due to Marriott's soft-landing elite policy.