Tall
boots may have a tab, loop or handle at the top known as a bootstrap, allowing one to use fingers or a
boot hook tool to help pulling the boots on. The
saying "to pull oneself up by one's bootstraps"
[ was already in use during the 19th century as an example of an impossible task. The
idiom dates at least to 1834, when it appeared in the
Workingman's Advocate: "It is conjectured that Mr. Murphee will now be enabled to hand himself over the Cumberland river or a barn yard fence by the straps of his boots."
In this case it refers to the process of creating the power supply for the high side MOSFET driver. The positive side of the upper driver power supply filter capacitor, which will power the floating high side MOSFET driver circuit, is charged through a blocking diode from a 12 to 15vdc supply when the bottom (lowside) MOSFET is ON. When the high side driver, now powered by the storage filter capacitor, drives the high side MOSFET ON, the high side driver power supply is jacked up along with the conduction of high side MOSFET. "jacked up" equals "bootstrapping" in this reference. You are pulling the high side driver supply (filter cap) "up by its bootstraps".
This method only worked when there is regular cycling of the output, like for a D.C. to A.C. power inverter. For long duration static, push-pull bridge motor direction control, this method will not work because the 'bootstrapped' filter cap which supplies power to the upper driver will eventually discharge beyond point of powering upper driver stage. The filter supply capacitor for the upper (high side) driver must be regularly refreshed, by low side MOSFET conduction, to provide enough supply voltage to power the upper, floating driver.
The floating upper driver gets its input logic control by a high voltage, output current sourcing input buffer. The source to drain breakdown voltage on this logic input transistor must be able to take the full voltage range. 400 to 600 volt breakdown voltage is common for a D.C. to A.C. power inverter.