So Slip Start is referring to turning off TC? That’s the first I’ve heard of that for the S. I thought it just allowed minor tire spin when moving from a full stop for use in snow, etcPage 58 of the manual shows a symbol that appears on your instrument cluster when TC kicks in.
Page 68 also tells you how it works, and using Slip Start to turn TC off.
Source = https://www.tesla.com/sites/default/files/model_s_owners_manual_north_america_en_us.PDF