Hey Rat,
There are several generations of Lincoln Tig 300/300's Meaning the High Frequency (HF) controls can be quite different from one unit to the next. But don't let that worry you.
All you need to do is find the "Code" number on the ID plate and then go here:
http://www.lincolnelectric.com/en-us/support/Pages/operator-manuals.aspx?type=text&q=TIG 300/300
The operators manuals are free for download.
I think you will find in most if not all units that the HF can be used in two different modes.. "Start" and "Continuous" The Start mode is for DC welding without a scratch start and Continuous is used for welding aluminum.
To your question: I'm also pretty sure that the HF in either mode is best used with a foot pedal (rheostat). The "contactor" that links to the foot pedal can also control the water for water cooled torches and the gas flow for inert gas welding. You are much better off having a foot pedal, though they may be hard to come by, Lincoln no doubt has some kind of adaptor out there so that newer pedals can be put to older machines...
What you DON'T want is the HF running when you are not trying to strike an arc.
The manual will also give you guidance for setting the HF "point gap" and how to control the intensity of the HF.
Let us know what you find out with your unit.
I used these in the Navy back in the Mid 80's for welding on A4's, A7's and FA/18's... They were the NEW and fancy machines at the time, replacing old P&H units.