President Donald Trump knows well that China will not honor the Phase I trade deal, because honoring it will lead to the CCP's end. The purpose of Trump's signing the deal is two-folded -- first, to counteract impeachment by the Democrats and to secure American farmers' votes; second, after winning the 2020 election, to legitimately crush China -- America's foremost threat as painted by VP Pence in 2018 -- when China fails to honor the deal. (Trump said he may wait till after 2020 election to finish phase two of US-China trade deal, and the tariffs on China goods will not be lifted until Phase II deal is reached.)
Indeed, there's no way China can increase purchases of US goods by $200B over 2 years, as attested by most financial experts (note 1). The deal also requires China to stop theft of US intellectual property and forced technology transfer from US companies, the 2 major means by which China has been able to progress in military affairs, science and technology. Opening up Chinese markets to US financial services firms will destroy China's state-capitalism, too.
Sooner or later, China will declare this deal to be unequal treaty that does not have to be honored, just like the 1842 Nanking Treaty by which Hong Kong was ceded to Britain and the 1901 Boxer Protocol, an aftermath of the 1900 Boxer Rebellion (note 2).
As stipulated in the deal, the enforcement mechanism allows the Trump administration to impose tariffs or other drastic measures within 90 days if officials determine that China isn't honoring the agreement.
Note 1: E.g., Alicia Garcia Herrero, Chief Economist Asia Pacific at Natixis in Hong Kong, Seng Yick Tee, an analyst at SIA Energy in Beijing, Iris Pang, Greater China economist at ING in Hong Kong, and Gavin Thompson, Vice Chair for Asia Pacific at Wood Mackenzie.
Note 2: According to Britta Redwood [2018], Legal Fellow at European Center for Constitutional and Human Rights e.V., "While unconscionability doctrine in contract law allows courts to deem contracts between unequal parties partially or totally unenforceable, international law treats sovereigns as equal parties and offers no such protection to weaker states."