As first reported by Reuters, the Pakistani Army is sending soldiers to Saudi Arabia on a "training and advice" mission.
This is notable because Pakistan has previously been reluctant to commit forces to Saudi military operations such as that in Yemen. Fearing a deterioration in relations with Iran (which is fighting the Saudis in Yemen right now) and being sucked into the Yemen quagmire, Pakistani Army headquarters had been happy to stay home (the Inter-Services Intelligence excluded), and the Pakistani Parliament deferred to that intent.
In turn, by now sending forces to Saudi Arabia at a time of unparalleled tensions between Riyadh and Tehran, the Pakistanis are centering themselves in the Saudi orbit. In specific terms, they are accepting that this deployment will lead to a deterioration in relations with Iran.
So, why the change of heart? While the Saudi and Pakistani governments did not immediately respond to requests for comment, I think two factors are at play here. First, the Pakistanis anticipate that Saudi crown prince Mohammed bin Salman will shortly take power from his father and want to ensure that he retains a pro-Pakistan attitude. Already the de facto Saudi leader, bin Salman is pursuing a bold and aggressive campaign of modernization. The Pakistani Army, which I've noted is now led by a more realistic commander, has obviously decided that it must keep bin Salman on-side. Not least among their calculations is Saudi Arabia's generous financial support to key elements of Pakistani national power.
There's also likely a personal element here. One of Pakistan's finest ever army commanders, Raheel Sharif is the head of a pan-Islamic-nation Saudi-funded counterterrorism force based in Riyadh. Sharif has won bin Salman's trust and wants Islamabad to cultivate the relationship further.
Second, the Saudis want to improve their capability to counter Iran. While the Saudi military is well-equipped with an array of high-end European and American military platforms, its combat forces lack the aggression and experience of the Iranians. Pakistan's battle-hardened army has the potential to whip the Saudis into shape. An apt military motto here: "If you're all kit (equipment), you're all shit."
Nevertheless, at the diplomatic level, this deployment is a pretty significant development. If nothing else, the Saudi-Pakistan alliance is growing closer and Iran more isolated, which is a good thing.