HubSpot's Collected Forms tool has the ability to capture form submissions, not only on your HubSpot (Lead Flow) form, but on most existing forms on your website too.
Collected Forms listens for a submission on a form on your website, which is identified by the HTML
<form> tag. As HubSpot detects a submission (bound to the form submission event), HubSpot gathers the information on the form and captures it in a cookie.
As soon as a submission has occurred, HubSpot stores the submission information in a cookie on the visitors browser. This cookie is called li_formsub. The purpose of this cookie is to ensure that HubSpot can process the submission, even if the form redirects to a new page.
In the event that the page redirects, once the contact loads a new page on your site, HubSpot will see the previously set li_formsub cookie and capture the information, removing the cookie in the process.
In the event that the page does not redirect, HubSpot will wait a brief moment and then capture the data stored in the li_formsub cookie and remove the cookie.
Once HubSpot captures the form submission from the cookie, it is processed by HubSpot, generating a submission notification email (if it is enabled) and pushing your contact to any connected email provider lists. This process can take approximately two minutes.