It's the 20 year rule, but luckily 1992 is outside it, so you're safe. I don't use it here, but PMR refers to Pridnestrovian Moldavian Republic, and is often used in the rhetoric about this.

Russia backed Trasnitria due to it having a high Slavic population, and the original reason that the 14th army (stationed in Moldova) provided backing to the separatists was to protect them from the ethnic chauvinism of the Moldovan government. Russia has a history of backing Slavic populations extraneous to their borders, so this is not an entirely new affair. The origins of this conflict lie in the Molotov-Ribbentrop pact, which allowed Stalin to annex Bessarabia, which is roughly where Moldova is now. They did this on the grounds that there was a seperate ethnic identity in Besserabia, which was being obfuscated by the "bourgeois nation" of Romania. Thus, the Soviet Union were liberating Besserabia from its oppressors, according to the propaganda.

This is where problems arise. The river Dniester splits Moldova into two parts, Transnitria and former Romanian Besserabia. Transnitria had an eighth of the territory, but was dominant in the economy/political system. The temperate climate also attracted many Soviet elite to holiday there. This left an uneasy stalemate and regional disparities until the Soviet Union began to collapse. In 1989, Moldovan and Romanian intellectuals pressured the Moldovan supreme soviet to move closer to its Romanian roots, and they changed the Moldovan alphabet to the Latin one. This ousted Russian as the de facto official language, and making the Moldovan alphabet "indistinguishable" from the Romanian one. This challenged the reasons for the Moldovan national identity, which was the idea that a separate ethnic group and culture existed, which Russia had created the state to protect. Under threat from pan-Romanian attitudes, the Transnitria elite appealed to Moscow for aid. Things began to escalate, and the Transnitrians seized weapons to defend against the "Romanian fascists", and declared a "Transnitrian Moldovan Republic". They were spurred on by the 14th Army, headquartered in Siraspol (in Transnitria), and in 1992, fierce fighting erupted along the Dniester river.

The reasons for Russian involvement are inherent in the rhetoric of the Transnitrians. As the fighting raged, their leaders accused the Moldovans of carrying out "genocide" against the ethnic Russian minority. The 14th army was ordered to stop the fighting (over fears the conflict might drag in the Ukraine/Romania), which was ironic considering that the army had fought for Transnitria, was supplying some weapons, and had publicly voiced support for their demands. This then lead to a cease in the conflict, and a heavy Russian peacekeeping force. There is debate to whether Moscow's encourage of the Transnitrians lead to an escalation of fighting, although many consider that it did. The main reason for intervening was to protect the interests of Russian speakers, although this position was undermined in 1994 by the fact that a quarter of the seats went to Yedinstvo (ultraconservative/Russian nationalist movement), who have distanced themselves from the old Transnitrian leadership. This was certainly influenced by the fact that of the 560,000 strong Russian minority at the time, less than a third lived in Transnitria. Sadly, I can't go any further, due to the 20 year rule, but that pretty much covers the reasons for Russian involvement.