Countries like the USSR will be influenced by events going on in the world, so Axis aggression/advances may trigger the USSR to swing towards the Allies, though your diplomatic events may help to counteract this, potentially delaying conflict with the USSR.
Typically, successes that didn't happen in real life that weaken the UK, such as Sealion or taking Egypt, will alert Stalin to the fact that the balance of power in Europe is swinging towards the Axis and he will need to step up military preparations/prepare for war.
But it's the sum of all the pushes and pulls in both directions that determine when they enter the war.
The USSR won't necessarily enter the war as a result of one action, it all depends on their Mobilization % at the time, because typically scripted events will move them a certain % towards/away from the Allies, and so if they already have a high mobilization % then they may enter the war in response to an Axis action, but if their mobilization level is low then this is very unlikely.
However, as their mobilization level increases so does their income, as they steadily gear up for war. This is where your diplomatic endeavours, particularly early in the game may help by reducing Soviet income over multiple turns, if it is successful.
I hope that helps?
It does help in that spending diplomacy points just delays the inevitable. Sooner or later the USSR/USA will declare war despite the fact I didn't do anything directly against them. As the leader of the third Reich I find their behavior despicable and traitorous. I would never do anything like that. Shame on them.